# Origin of Vertebrates/Chapter IX

CHAPTER IX
THE PROSOMATIC SEGMENTS OF AMMOCŒTES
The prosomatic region in Ammocœtes.—The suctorial apparatus of the adult Petromyzon.—Its origin in Ammocœtes.—Its derivation from appendages.—The segment of the lower lip or metastomal segment.—The tentacular segments.—The tubular muscles.—Their segmental arrangement.—Their peculiar innervation.—Their correspondence with the system of veno-pericardial muscles in Limulus.—The old mouth or palæostoma.—The pituitary gland.—Its comparison with the coxal gland of Limulus.—Summary.
In the last chapter it was seen not to be incompatible with both the anatomical and morphological evidence to look upon the trigeminal nerves as having originally supplied the seven prosomatic pairs of appendages of the invertebrate ancestor, the foremost of which, the cheliceræ, and the four pairs of endognaths dwindled away and became insignificant, leaving as trace of their former presence the descending root of the Vth nerve; while the two hindmost pairs, the ectognaths and the chilaria, or metastoma, remained vigorous and developed, leaving as proof of their presence the nucleus masticatorius. Evidence in favour of this suggestion and of the nature of the dwindling process is afforded when we examine what the trigeminus does supply in Ammocœtes. In all vertebrates this nerve supplies the great muscles of mastication which, in all gnathostomatous fishes, move the jaws. The lowest fishes, the cyclostomes, possess no jaws; they take in their food by attaching themselves to their prey and by means of rasping teeth situated in serried rows within the circular mouth, combined with a powerful suctorial apparatus, they suck the juices of the fish they feed upon. Not possessing jaws, they feed by suction on the living animal, a method of feeding which gives them no more claim to be classed as parasitic animals than the whole group of spiders which feed in a similar manner on living flies.
The Origin of the Suctorial Apparatus of Petromyzon.
This powerful suctorial apparatus is innervated entirely by the trigeminal nerve, so that here in its muscular arrangements any original segmental arrangement of the muscles of mastication might be expected to be visible. It consists of a large rod or piston, to which are attached powerful longitudinal muscles; a large muscle, the basilar muscle, which assists the piston in producing a vacuum, and annular muscles around the circular lip.
Turn now to the full-grown larval form, Ammocœtes, an animal in the case of Petromyzon Planeri as large as the full-grown Petromyzon, and seek for this musculature. There is, apparently, no sign of it, no suctorial apparatus whatever, only, as already mentioned, an oral chamber bounded by the lower and upper lips and the remains of the septum between it and the respiratory chamber—the velar folds. Attached to its walls a number of tentacles are situated, which form a fringe around and within the mouth. Most extraordinary is the contrast here between the larval and the adult stages; in the former, no sign of the suctorial apparatus, but simply tentacles and velar folds; in the latter, no sign of tentacles or of velar folds, but a massive suctorial apparatus.
In order, then, to understand the origin of the muscles of mastication, it is necessary to study the changes which occur at transformation, and thus to find out how the suctorial apparatus of the adult arises. This most important investigation has been undertaken by Miss Alcock, and owing to the kindness of Mr. Millington, of Thetford, we have been able to obtain a better series in the transformation process than has ever been obtained before. Miss Alcock has not yet published her researches, but has allowed me to make use of some of her facts.
An enormous proliferation of muscular tissue takes place with great rapidity during this transformation, which causes the disappearance of the tentacles, and gives origin to the suctorial apparatus. The starting point of this proliferation can be traced back in all cases to little groups of embryonic tissue found below the epithelial lining of the oral chamber in Ammocœtes. Of these groups the most conspicuous one is situated at the base of the large median ventral tentacles. Others are situated at the base of the tentacular ridge. Further, although this extraordinary change takes place in the ​peripheral organ, no marked difference occurs in the arrangement of the nerves issuing from the trigeminal motor centre, no new nerves are formed to supply the new muscles, but every motor nerve-fibre and the motor cell from which it arises increases enormously in size, and these giant nerve-fibres thus formed split into innumerable filaments corresponding with the proliferation of the muscular elements.
The clue, then, to the origin of the suctorial apparatus and of the nature of the original organs supplied by the trigeminal is afforded in this case, as in all other similar inquiries, by the central nervous system and its outgoing nerves. Here is always the citadel, the fixed seat of government, here is 'headquarters,' from which the answers to all our inquiries must originate.
Fig. 114.—Distribution of Trigeminal Nerve in Ammocœtes.
The Trigeminal Nerve of Ammocœtes.
Striking is the answer. In Fig. 114, Miss Alcock has drawn the distribution of the trigeminal nerve as traced by her through a series of sections. It arises, as is well known, from two separate ganglia, of which the foremost gives rise to a purely cutaneous nerve, the ophthalmic nerve, and the hindmost to three nerves, the most posterior of which is purely cutaneous and passes tailwards over the ventral branchial region, as shown in the figure; the other two nerves, both ​of which contain motor fibres, are called by Hatschek the mandibular and maxillary nerves. Of these the mandibular or velar nerve (met.) is a large, conspicuous nerve, which arises so separately from the rest of the trigeminal as almost to deserve the title of a separate nerve. When it leaves the large posterior ganglion, it passes into the anterior part of the velum, runs along with the tubular muscles, which it supplies, to the ventral surface as far as the junction of the lower lip with the thyroid plate, and has not been followed further by Hatschek. Miss Alcock, however, by means of serial sections, has traced it further, and shown that at this point it turns abruptly headwards to terminate in the muscles of the lower lip. If, then, as suggested, the lower lip represents the metastoma—the last pair of prosomatic appendages—then this mandibular or velar nerve represents that segmental nerve.
The other nerve—the maxillary nerve of Hatschek—which constitutes the larger part of the trigeminal, passes forwards from the ganglion, and at a point somewhere about the anterior region of the eyeball, divides into two, an external (black in Fig. 114) and an internal (red in Fig. 114) nerve. The external branch is apparently entirely sensory, and supplies the external surfaces of the upper and lower lips. The internal branch is mainly motor, and supplies the muscles of the upper lip; it contains also the nerves of the tentacles.
The nerve to the median ventral tentacle (t.) or tongue leaves the internal division of the maxillary immediately after its separation from the external; it runs ventralwards, and at the same time passes internally until it reaches a position between the muco-cartilage and the epithelium lining the cavity of the throat. It then turns, and passing posteriorly (towards the tail) to the point where the median ventral tentacle is attached to the lower lip, it supplies some very rudimentary-looking muscles which run from the tentacle to the adjoining surface, and no doubt serve to move the tentacle from side to side. A portion of the nerve still continues to run along the side of the median ventral ridge, as far back as the point where the muscles of the hyoid segment pass round to the ventral side between the velum and the thyroid; in fact, this small nerve passes along the whole length of the median ventral ridge.
This description shows that the trigeminal nerve divides itself into two groups: the one represented black in the figure, which is purely cutaneous and sensory, corresponding, in the main, according ​to my theory, to the epimeral nerves of Limulus; the other coloured red, which supplies muscles belonging to the visceral or splanchnic muscle-group, and contains also the nerves to the tentacles.
This latter group, which is formed by two distinct well-defined nerves, viz. the mandibular and the internal branch of the maxillary, corresponds, according to my theory, to the amalgamated nerves of the prosomatic appendages, and is clearly divisible into three distinct nerves—
1. The lower lip-nerve or the metastomal nerve (met.).
2. The tongue-nerve (t.).
3. The nerve (tent.) to the upper lip and tentacles.
Of these three pairs of nerves it is suggested that the first pair were derived from the nerves to the metastomal appendage. The second pair of nerves ought, on this theory, originally to have supplied the pair of appendages immediately in front of the metastoma—that is, the pair of ectognaths, and therefore the ventral pair of tentacles, known as the tongue, would represent the last remnant of these ectognaths. Similarly, the other tentacles would represent the endognaths, and therefore the third pair of nerves would represent the fused nerves to these concentrated endognaths, which, in the Eurypterids, stand aloof from the ectognaths.
Let us consider these three propositions separately. In the first place, have we any right to attribute segmental value to the mandibular nerve? What evidence is there of segments in this region in Ammocœtes?
The Segment of the Lower Lip, or Metastomal Segment.
We have seen that in the branchial or mesosomatic region the segments corresponding to the mesosomatic appendages were mapped out by means of their supporting or skeletal structures, their segmental muscles, and their nervous arrangements, as well as by the arrangement of the branchiæ. Similarly, the segments in front of the branchial region, corresponding to the prosomatic appendages, ought to be definable by the same means, although, owing to the absence of branchiæ and the greater concentration in this region, the separate segments would probably not be so conspicuous.
The last segment considered was the segment belonging to the VIIth nerve corresponding to the opercular appendages of the ​Eurypterid. The segment immediately in front of this is the next for consideration, viz. that corresponding to the chilarial appendages or metastoma; and as the basal part of this pair of appendages was fused with the basal part of the operculum, the one cannot be discussed without the other; therefore, the segment to which the lower lip belongs must be considered in connection with and not apart from the thyro-hyoid segments already dealt with.
In Chapter V., p. 188, I stated that the supporting bars of the foremost mesosomatic segments, the thyro-hyoid segments, differed from the cartilaginous bars of the branchial segments, in that they were composed of muco-cartilage. Also in addition to the muco-cartilaginous skeletal bars, a ventral plate of muco-cartilage exists in Ammocœtes which covers over the thyroid gland.
Similarly in the prosomatic segments the skeletal bars are composed of muco-cartilage and the ventral plate of muco-cartilage continues forward as the plate of the lower lip. It is of special interest, in connection with the segments indicated by such supporting structures, to find that this special tissue is entirely confined to the head-region, and disappears absolutely at transformation, thus indicating the ancestral nature of the segments marked out by its presence.
This muco-cartilaginous skeleton is the key to the whole position, and requires, therefore, to be understood. It is of great importance, not only because it demonstrates the position of the segments in Ammocœtes which characterized its invertebrate ancestor, but also because it possesses a structure remarkably similar to that found in the head-plates of the most ancient fishes. For the present I will confine myself to the consideration of this muco-cartilaginous skeleton as evidence of the relationship of Ammocœtes to the Eurypterids, and in the next chapter will show how absolutely the same skeleton corresponds to that of the Cephalaspidæ, so that Ammocœtes is really a slightly modified Cephalaspid, the larval form of which was Eurypterid in character.
Fig. 115.—Dorsal half of Head-region of Ammocœtes.
Fig. 116.—Horizontal Section through the Anterior Part of Ammocœtes, immediately Ventrally to the Auditory Capsule.
Fig. 117.—Sagittal Lateral Section through the Anterior Part of Ammocœtes.
Lettering and colouring same as in Fig. 116. aud., auditory capsule; j.v., jugular vein.
In Chapter IV., Figs. 63, 64, I have given a representation of the ventral and dorsal views of an Ammocœtes cut in half horizontally. Such a section shows with great clearness the series of branchial appendages with their segmental muscles and cartilaginous bars which form the branchial segments innervated by the IXth and Xth nerves, according to my view of the branchial unit. As is seen (Fig. 64 or 115), the skeletal bar of the hyoid or opercular appendage, which is clearly serially homologous with the other branchial bars, is composed of muco-cartilage, and not of cartilage. If we follow this series of horizontal sections nearer to the origin of the cartilaginous bars from the sub-chordal cartilaginous rod on each side of the notochord, we obtain a picture, as in Fig. 116, in which each branchial segment is defined by the section of the branchial cartilaginous bar(sk$_4$, sk$_5$), by the section of the separate branchiæ (br$_2$, br$_3$), and by the separate segmental muscles arranged round each bar, these muscles being partly ordinary striated (m$_4$, m$_5$), partly tubular (mt$_3$, mt$_4$). The uppermost of these branchial segments shows the same arrangement; (sk$_3$) is the branchial skeletal bar, which is now composed of muco-cartilage, not cartilage; (br$_1$) is the branchiæ in the same situation as the others, but here composed of glandular rather than of respiratory epithelium, while the ordinary striated branchial muscles of this segment are marked as (m$_3$), being separated from the tubular muscles of the segment (mt$_2$), owing to the large size of the blood-space in which these latter muscles are lying. In front of this segment so defined we see again another well-marked skeletal bar (sk$_2$) of muco-cartilage, evidently indicating a similar segment anterior to the hyoid segment. In connection with this bar there are no branchiæ, but again we see two sets of visceral muscles, the one ordinary striated, marked (m$_2$), and the other tubular, marked (mt$_1$). Here, then, the section indicates the existence of a segment of the same character as the posteriorly situated branchial segments but belonging to a non-branchial region—a segment which would represent a non-branchial appendage, the last, therefore, of the prosomatic appendages. Let us, then, follow ​out these two segmental muco-cartilaginous bars and their attendant muscles, and see to what sort of segments their investigation leads.
The bar which comes first for consideration (sk$_3$) arises immediately behind the auditory capsule from the first branchial cartilage very soon after it leaves the sub-chordal cartilaginous ligament; the soft cartilage of the sub-chordal ligament ceases abruptly in its extension along the notochord at the place where the hard cartilage of the parachordal joins it, and in a sense it may be said to leave the notochord at this place and pass into the basal part of the first branchial bar. The most anterior continuation of this branchial system is this muco-cartilaginous bar (sk$_3$), which passes forward and ventralwards, being separated from the axial line by the auditory capsule (cf. Fig. 118, A, B, C). Its position is well seen in a sagittal section, such as Fig. 117. It follows absolutely the line of the pseudo-branchial groove (ps. br., Fig. 114), and ventrally joins the plate of muco-cartilage which covers the thyroid gland. It forms a thickened border to this plate anteriorly, just as the branchial cartilaginous bars border it posteriorly. In fact, it behaves with respect to the hyoid segment in a manner similar to the rest of the cartilaginous bars with respect to their respective segments.
It represents, although composed of muco-cartilage, the cartilaginous bar of the operculum in Limulus, which also forms the termination of the branchial cartilaginous system, as fully explained in Chapter III.; it may therefore be called the opercular bar.
The next bar (sk$_2$) is extremely interesting, as we are now out of the branchial or mesosomatic region, and into the region corresponding to the prosoma. It starts from a cartilaginous projection made of hard cartilage, just in front of the auditory capsule, called by Parker the 'pedicle of the pterygoid'—a projection (ped.) which defines the posterior limit of the trabeculæ on each side, where they join on to the parachordals,—and winding round and below the auditory capsule, joins the opercular bar (cf. Fig. 118), to pass thence into and form part of the muco-cartilaginous plate of the lower lip. In the section figured (Fig. 116), this projection of hard cartilage is not directly continuous with (sk$_2$), owing to a slight curvature in the bar; the next few sections show clearly the connection between (ped.) and (sk$_2$), and consequently the complete separation by means of this bar of the hyoid segment from the segment in front.
Fig. 118.—Skeleton of Head-Region of Ammocœtes. A, Lateral View; B, Ventral View; C, Dorsal View.
​In the figures, the hard cartilage is coloured purple, the soft cartilage blue, and the muco-cartilage red, so that the position of this bar is well shown. This bar may be looked upon as bearing the same relation to the muco-cartilaginous plate of the lower lip as the opercular bar does to the muco-cartilaginous plate over the thyroid; and seeing that these two plates form one continuous ventral head-shield of muco-cartilage (Fig. 118, B), and also that this bar fuses with the opercular bar, we may conclude that the segment represented by the lower lip is closely connected with the hyoid or opercular segments. In other words, if the lower lip arose from the metastoma, then this pair of skeletal bars might be called the metastomal bars, which formed the supporting skeleton of the last pair of prosomatic appendages and, as is likely enough, arose in connection with the posterior lateral horns of the plastron; these posterior lateral horns, like the rest of the plastron, would give rise to hard cartilage, and so form in Ammocœtes the two lateral so-called pterygoid projections.
In the branchial region the muscles which marked out each branchial segment were of two kinds—ordinary striated visceral muscles and tubular muscles. Of these the former represented the dorso-ventral muscles of the branchial appendages, while the latter formed a separate group of dorso-ventral muscles with a separate innervation which may have been originally the segmental veno-pericardial muscles so characteristic of Limulus and the scorpions. In Figs. 116, 117, the grouping of these muscles in each branchial segment is well shown, and it is immediately seen that the hyoid segment possesses its group of striated visceral muscles (m$_3$) supplied by the VIIth nerve in the same manner as the posterior groups, as has already been pointed out by Miss Alcock in her previous paper. Passing to the segment in front, Fig. 116 shows that the group of visceral muscles (m$_2$) corresponds in relative position with respect to the metastomal bar to the hyoid muscles with respect to the opercular bar or to the branchial visceral muscles with respect to each branchial bar. What, then, is this muscular group? The series of sections show that these are the dorso-ventral muscles belonging to the lower lip, which, as seen in Fig. 119 (M.), form a well-marked muscular sheet, whose fibres interlace across the mid-ventral line of the lower lip. This group of lower lip-muscles is very suggestive, for these muscles arise, not from the trabeculæ, but from the front dorsal region of the cranium, just in front of the two lateral ​eyes. In Fig. 117 the dorsal part is seen cut across on its way to its dorsal attachment. Such an origin is reminiscent of the tergo-coxal group of muscles, arising, as they do, from the primordial cranium and the tergal carapace, and suggests at once that when the chilarial appendages expanded to form a metastoma, their tergo-coxal muscles formed a sheet of muscles similar to those of the lower lip of Ammocœtes, by which the movements of the metastoma were effected. The posterior limit of these muscles ventrally marks out the junction of the segment of the lower lip with that of the thyroid; in other words, indicates where the metastoma had fused ventrally with the operculum (Fig. 117).
Fig. 119.—Ventral View Of Head-Region of Ammocœtes.
Th., thyroid gland; M., lower lip, with its muscles.
Besides the striated visceral muscles, each branchial segment possesses its own tubular muscles, shown in Fig. 116 (mt$_3$) and (mt$_4$). As the section shows, there is clearly a group of tubular muscle-fibres belonging to the hyoid segment (mt$_2$), and also another group belonging to the segment in front of the hyoid (mt$_1$); so that, judging from this section, each of these segments possesses its own tubular musculature just as do the branchial segments, the difference being that the tubular muscles are more separated from the striated visceral group than in the true branchial segments, owing to the size of the blood-spaces surrounding them. What, then, are these two groups of muscles? Tracing them in the series of sections, both groups are seen to belong to the system of velar muscles, forming an anterior and a posterior group respectively; and we see, further, that there is not the slightest trace of any tubular muscles anterior to these muscles of the velum.
In the living Ammocœtes the velar folds on each side can be seen ​to move synchronously with the movements of respiration, contracting at each expiration, and thus closing the slit by which the oral and respiratory chambers communicate, and so forcing the waters of respiration through the gill-slits, as described by Schneider. Such a fact is clear evidence that these tubular muscles of the velar folds belong to the same series as the tubular muscles of the branchial segments, so that if, as I have already suggested, the latter muscles were originally the veno-pericardial muscles of segments corresponding to the branchial appendages, then the former would represent the veno-pericardial muscles of the segments corresponding to the opercular and metastomal appendages. What, then, are these velar folds, and how is it that the tubular muscles of these two segments become the velar muscles? I will consider, in the first instance, the posterior group of muscles (mt$_2$) in Fig. 116.
It has already been pointed out that the tubular muscles of the branchial segments are dorso-ventral, but do not run with the ordinary constrictors, having separate attachments and running part of their course internally to and part externally to the ordinary constrictors. At first sight, as is usually stated, the hyoid segment does not appear to possess tubular muscles at all. If, however, we follow the posterior group of velar muscles (mt$_2$), we see (Fig. 117) that they pass between the auditory capsule and the opercular bar (sk$_3$) of muco-cartilage to reach the region of the jugular vein (j.v.) posteriorly to the auditory capsule, so that their dorsal origin bears the same relation to the hyoid segment as the dorsal attachment of the rest of the tubular muscles to their respective segments. Further, these muscles run along the length of the velar fold, and are attached ventrally on each side of the thyroid gland, so that their ventral attachment also corresponds in position, as regards the hyoid segment, with the ventral attachment of the rest of the tubular muscles as regards their respective segments.
This ventral attachment is shown in Fig. 119 on each side of the thyroid, and in Fig. 120 (mt$_2$); while in Fig. 117 the fibres are seen converging to this ventral position. In other words, this large posterior muscle of the velar folds is a dorso-ventral muscle, and would actually take the same position in the hyoid segment as the dorso-ventral tubular muscles in the other branchial segments, if the velum were put back into its original position as the septum terminating the branchial chamber. Conversely, the presence of these ​hyoid tubular muscles in the velum gives evidence that the opercular segment takes part in the formation of the septum, as already suggested.
Miss Alcock, in her paper, speaks of tubular muscles belonging to the hyoid segment, which are attached to the muco-cartilage. Schaffer also speaks of certain tubular muscles belonging to the velar group as piercing the muco-cartilage (h. r. s.) in his figures 24 and 25, i.e. the metastomal bar, near its junction with the opercular bar. In my specimens there is a distinct group of tubular muscles which pierce the opercular bar of muco-cartilage at its junction with the metastomal bar, and pass into the posterior group of velar muscles. They clearly belong to the hyoid segment, as Miss Alcock supposed, but are not attached to the muco-cartilage. It is possible that they represent a different group to those already considered, and suggest the possibility that this opercular or thyro-hyoid segment is double with respect to its original veno-pericardial muscles as well as in other respects.
The anterior group of tubular muscles (mt$_1$, Figs. 116, 117) belonging to the same segment as the metastomal bar must now be taken into consideration. Very different is their origin to that of the posterior group: they arise close up against the eye, and have given rise to Kupffer's and Hatschek's misconception that the superior oblique muscle of the eye arises from a part of the velar musculature. Naturally, as Neal has pointed out, they have nothing to do with the eye-muscles; the superior oblique muscle is plainly in its true place entirely apart from these velar muscles, which form the foremost group of the segmental tubular muscles. They pass into the anterior part of the velar folds and run round to the ventral side just in the same way as does the posterior group. This anterior group of tubular muscles represents the veno-pericardial muscle of the segment immediately in front of the opercular, i.e. the metastomal segment, and is the foremost of these veno-pericardial muscles. Its presence shows that the velar folds, formed as they were by the breaking down of the septum, are in reality part of two segments, viz. the opercular and the metastomal, which have fused together in their basal parts, and by such fusion have caused the inter-relationship between the VIIth and Vth nerves, so apparent in the anatomy of the vertebrate cranial nerves.
A further piece of evidence that this anterior portion of the velum ​belongs to the same segment as the lower lip is the fact that in addition to the tubular muscles a single ordinary striated muscle is found in the velum which, like the muscles of the lower lip, is innervated by this same mandibular nerve.
This muscle is attached laterally to the muco-cartilage of the metastomal bar (sk$_2$) at its junction with the muco-cartilage of the lower lip, and spreads out into a number of strands which are attached at intervals along the whole length of the free anterior edge of the velum. It is the only non-tubular muscle belonging to the velum, and by its contraction it draws the anterior portions of the velar folds apart from each other, and so opens the slit between them, through which the food and mud must pass. Clearly from its position it does not belong to the original tergo-coxal group of muscles as do those of the lower lip; it must have been one of the intrinsic muscles of the metastoma itself.
This anterior portion of the velar folds affords yet another striking hint of the correctness of my comparison of the lower lip segment of Ammocœtes with the chilaria of Limulus or the metastoma of Eurypterus; for the most dorsal anterior portion, which at its attachment possesses a wedge of muco-cartilage, forms a separate, well-defined, rounded basal projection marked Ser. in Fig. 115, and B in the accompanying Fig. 120. This is that part of the velar folds which comes together in the middle line and closes the entrance into the respiratory chamber. The epithelial surface here is most striking and suggestive, for it is markedly serrated, being covered with a large number of closely-set projections or serræ. The serration of the surface here is of so marked a character that Langerhans considered this part of the velar folds to act as a masticating organ, grinding and rasping the food and mud which passed through the narrow slit. In fact, Langerhans supposed that this portion of the velum acted in a manner closely resembling the action of the gnatho-bases of the prosomatic appendages in Limulus or the Eurypteridæ.
This suggestion of Langerhans is surely most significant, considering that this somewhat separate portion of the velum, to which he assigns such a function, is in the very place where the gnathite portion of the metastomal appendages would have been situated if it were true that the lower lip and anterior portion of the velum of Ammocœtes were derived from the metastoma.
In addition to this marked serrated edge the whole surface of ​the anterior portion of the velum is covered over with a scale-like or tubercular pattern remarkably like the surface-ornamentation seen in many of the members of the ancient group Eurypteridæ. In Fig. 121 I give a picture of this surface-marking of the velum. It is striking to see that just as in the case of the invertebrate this marking and these serræ are formed simply by the cuticular surface of the epithelial cells; a surface which, according to Wolff, possibly contains chitin. The interpretation which I would give of the velar folds is therefore as follows:—
They represent the fused basal parts of the opercular and metastomal appendages, the gnatho-bases of the latter still retaining in a reduced degree their rasping surfaces, because, owing to their position on each side of the opening into the respiratory chamber they were still able to manipulate the food as it passed by them after the closure of the old mouth.
Fig. 120.—Ammocœtes cut open in Mid-Ventral Line to show Position of Velum; Velar Folds removed on one side.
The whole evidence points irresistibly to the conclusion that the mandibular or velar nerve of the trigeminal does supply a splanchnic ​segment which is, in all respects, comparable with the segments supplied by the facial, glossopharyngeal, and vagus nerves, except that it does not possess branchiæ. This simply means that the appendages which these nerves originally supplied were prosomatic, not mesosomatic, and corresponded, therefore, to the chilarial or metastomal appendages.
A comparison of the ventral surface of Slimonia, as given in Fig. 8, p. 27, with that of Ammocœtes (Fig. 119), when the thyroid gland and lower lip muscles have been exposed to view, enables the reader to recognize at a glance the correctness of this conclusion.
The Tentacular Segments and the Upper Lip.
Anterior to this metastomal segment, Fig. 116 shows a group of visceral muscles, m$_1$, and yet again a muco-cartilaginous bar, sk$_1$, but, as already stated, no tubular muscles. These visceral muscles indicate the presence in front of the lower lip-segment of one or more segments of the nature of appendages. The muscles in question (m$_1$) are the muscles of the upper lip, the skeletal elements form a pair of large bars of muco-cartilage (sk$_1$), which start from the termination of the trabeculæ, and pass ventralwards to fuse with the muco-cartilaginous plate of the lower lip (Figs. 117 and 118). This large bar forms the tentacular ridge on each side, and gives small projections of muco-cartilage into each tentacle. In addition to this tentacular bar, a special bar of muco-cartilage exists for the fused pair of median tentacles, the so-called tongue, which extends in the middle line along the whole length of the lower lip, being separated from the muco-cartilaginous plate of the lower lip by the muscles of the lower lip. This tongue bar of muco-cartilage joins with the muco-cartilage of the lower lip at its junction with the thyroid plate, and also with the tentacular bar just before the latter joins the muco-cartilaginous plate of the lower lip. This arrangement of the skeletal tissue suggests that the pair of tentacles known as the tongue stand in a category apart from the rest of the tentacles; a suggestion which is strongly confirmed by the separate character of its nerve-supply, as already mentioned.
For three reasons, viz. the separateness both of their nerve-supply and of their skeletal tissue, and the importance they assume at transformation, this pair of ventral tentacles must, it seems to me, be put ​into a separate category from the rest of the tentacles. On the other hand, the innervation of the rest of the tentacles by a single nerve which sends off a branch as it passes each one, together with the concentration of their skeletal elements into a single bar, with projections into each tentacle, points directly to the conclusion that these tentacles must be considered as a group, and not singly.
I suggest that these tentacles are the remains of the ectognaths and endognaths; the tongue representing the two ectognaths, and the four tentacles on each side the four pairs of endognaths.
As we see, this method of interpretation attributes segmental value to the tentacles, a conclusion which is opposed to the general opinion of morphologists, who regard them as having no special morphological importance, and certainly no segmental value. On the other hand, the importance of the pair of ventral tentacles, the 'tongue' of Rathke, which lie in the mid-line of the lower lip, has been shown by Kaensche, Bujor, and others, all of whom are unanimous in asserting that at transformation they are converted into that large and important organ the piston or tongue of the adult Petromyzon. It is supposed that the rest of the tentacles vanish at transformation, being absorbed; they appear to me rather to take part in the formation of the sucking-disc, so that I am strongly inclined to believe that the whole of the remarkable suctorial apparatus of Petromyzon is derived from the tentacles of Ammocœtes. In other words, on my view, a conversion of the prosomatic appendages into a suctorial apparatus takes place at transformation, just as is frequently the case among the Arthropoda.
It is to the arrangement of the muscles that we look for evidence of segmental value. As long as it was possible to look upon these tentacles as mere sensory feelers round the mouth entrance, it was natural to deny segmental value to them. Matters are now, however, totally different since Miss Alcock's discovery of the rudimentary muscles at the base of the tentacles and their development at transformation. If these muscles represent some of the appendage muscles belonging to the foremost prosomatic segments just as the ocular muscles represent the dorso-ventral somatic muscles of those same segments, then we may expect ultimately to be able to give as good evidence of segmentation in their case as I have been able to give in the case of these latter muscles; for the two sets of muscles are curiously alike, seeing that the eye-muscles do not develop until ​transformation, but throughout the Ammocœtes stage remain in almost as rudimentary a condition as the tentacular muscles.
Another difficulty with respect to the tentacles is the determination of the number of them, owing to the fact that in addition to what may be called well-defined tentacles a large number of smaller tactile projections are found on the surface of the upper lip, as is seen in Fig. 115. In the very young condition, 7 or 8 mm. in length, it is easier to make sure on this point. At this stage they may be spoken of as arranged in two groups: an anterior small group and a posterior larger group. The anterior group consists of a pair of very small tentacles and a very small median tentacle, all three situated quite dorsally in the front part of the upper lip. The posterior group, which is separate from the anterior, consists of five pairs of much larger tentacles, the most ventral pair in the mid-line ventrally on the lower lip being fused together to form the large ventral median tentacle or tongue already mentioned. This pair, according to Shipley, is markedly larger than the others. There are, therefore, five conspicuous tentacles on each side, and in front of them a smaller pair and a small median dorsal one. In the very young condition the accessory projections above-mentioned are not present, or at all events are not conspicuous, and the tentacles are also markedly larger in comparison to the size of the animal than in the older condition, where they have distinctly dwindled.
This posterior group of five conspicuous tentacles is the one which I suggest represents the four endognaths and one ectognath. What the significance of the small anterior group is, I know not. It is possible that the cheliceræ are represented here, for they are situated distinctly anterior to the other group; I know, however, of no sign of a markedly separate innervation to these most dorsal tentacles such as I should have expected to find if they represented the cheliceræ.
The muscles of the upper lip, which distinctly belong to the visceral and not to the somatic musculature, form part of the foremost segments, and in these muscles the tentacular nerve reaches its final destination. From their innervation, then, they must have belonged to the same appendages as the tentacles supplied by the tentacular nerve, i.e. to the endognaths. What conclusion can we form as to the probable origin of the upper lip of Ammocœtes? Since the oral chamber was formed by the forward growth of the metastoma, i.e. the lower lip of Ammocœtes, it follows that the upper ​lip is the continuation forwards of the original ventral surface of such an animal as Limulus or a member of the scorpion group, where there is no metastoma, and corresponds to the endostoma, as Holm calls it, of Eurypterus. This termination of the ventral surface in all these animals is made up of two parts: (1) Of sternites composing the true median ventral surface of the body, called by Lankester the pro- and meso-sternites; and (2) of the sterno-coxal processes of the foremost prosomatic appendages, called in the case of Limulus gnathites, because they are the main agents in triturating the food previously to its passage into the mouth. In Limulus, a conjoined pro-mesosternite forms the median ventral wall to which the sterno-coxal processes are attached on each side, and in Phrynus and Mygale a well-marked pro-sternite and meso-sternite are present, forming the posterior limit of the olfactory opening. In Buthus and the true scorpions the sterno-coxal processes of the 2nd, 3rd, and 4th prosomatic appendages take part in surrounding the olfactory tubular passage; in Thelyphonus only the processes of the 2nd pair of prosomatic appendages play such a part, the pro-sternite not being present (cf. Fig. 97).
Seeing, then, what a large share the sterno-coxal processes of one or more of these prosomatic appendages plays in the formation of this endostoma, and seeing also that the nerve which supplies the upper lip-muscles in Ammocœtes is the same as that supplying the tentacles which are attached to the upper lip, it appears to me more probable than not that the muscles in question are the vestiges of the sterno-coxal muscles. These muscles differ markedly in their attachments from the muscles of the lower lip, for whereas the latter resemble the tergo-coxal group in their extreme dorsal attachment, the former resemble the sterno-coxal group in their attachment to what corresponds to the endostoma.
This interpretation of the meaning of the transformation process is in accordance with all the previous evidence both from the side of the palæostracan as from the side of the vertebrate, for it signifies that a dwindling process has taken place in the foremost of the original prosomatic appendages—the cheliceræ and the endognaths; while, on the contrary, the ectognath and the metastoma have continued to increase in importance right into the vertebrate stage. This process is simply a continuation of what was already going on in the invertebrate stage, for whereas in Eurypterus and other cases ​the cheliceræ and endognaths had dwindled down to mere tentacles, the ectognath was the large swimming appendage, and the metastoma was on the upward grade from the two insignificant chilaria of Limulus.
The transformation of these foremost appendages into a suctorial apparatus is very common among the arthropods, as is seen in the transformation of the caterpillar into the butterfly, and it is in accordance with the evidence that the main mass of that suctorial apparatus should be formed from appendages corresponding to the ectognath and metastoma rather than from the four endognaths. In all probability the nucleus masticatorius of the trigeminal nerve with its innervation of the great muscles of mastication is evidence of the continued development of the musculature of these two last prosomatic appendages, just as the descending root of the Vth demonstrates the further disappearance of all that belongs to the foremost prosomatic appendages. As yet, however, as far as I know, the musculature of the head-region of Petromyzon has not been brought into line with that of other vertebrates, and until that comparative study has been completed it is premature to discuss the exact position of the masticating muscles of the higher vertebrates.
The analysis of these tentacular segments belonging to the trigeminal nerve presents greater difficulties than that of any of the other cranial segments, owing to the deficiency of our knowledge of what occurs at transformation. Light is required not only on the origin of the new muscles but also on the origin of the new and elaborate cartilages which are newly formed at this time.
Miss Alcock has not yet worked out the origin of all these cartilages and muscles, so that we are not yet in a position to analyze the trigeminal supply in Petromyzon into its component appendage elements, an analysis which ought ultimately to enable us to determine from which appendage-muscles the masticating muscles in the higher vertebrates have arisen. As far as the muscles are concerned, she gives me the following information:—
The tongue-nerve supplies in Ammocœtes the rudimentary muscles which pass laterally from the base of the large ventral tentacle to the wall of the throat, and even in Ammocœtes must possess some power of moving that tentacle.
At transformation these muscles proliferate and develop enormously, and form the bulk of the large basilar muscle which ​surrounds the throat ventrally and laterally, and is the most bulky muscle in the suctorial apparatus.
The velar or mandibular nerve supplies in Ammocœtes the muscles of the lower lip. In Petromyzon it supplies also the longitudinal muscles of the tongue. The tongue-cartilage first develops in the region of the median ventral tentacle, and there the longitudinal tongue-muscles first begin to develop, not from the rudimentary muscles in the tongue but from those in the lower lip region.
In Ammocœtes the tentacular nerve supplies the rudimentary muscles in the tentacles and the muscles of the upper lip. The latter disappear entirely at transformation, and in Petromyzon the tentacular nerve supplies the circular, pharyngeal, and annular muscles, which are derived from the rudimentary tentacular muscles.
For the convenience of my reader I append here a table showing my conception of the manner in which the endognathal and ectognathal segments of the Palæostracan are represented in Ammocœtes. It shows well the uniform manner in which all the individual segmental factors have been fused together to represent the appearance of a single segment (van Wijhe's first segment) in the case of the four endognathal segments, but have retained their individuality in the case of the ectognathal segment.
| V. Wijhe's segments. | Eurypterid segments. | Eurypterid segments. | Appendages. | Appendages. | Appendage nerves. | Skeletal elements. | Somatic motor nerves. | Dorso- ventral segmental muscles. | Cœlomic cavities. | Coxal glands. |
| V. Wijhe's segments. | Eurypterid segments. | Eurypterid segments. | Eurypterid. | Ammocœtes. | Appendage nerves. | Skeletal elements. | Somatic motor nerves. | Dorso- ventral segmental muscles. | Cœlomic cavities. | Coxal glands. |
| 1 | 2 3 4 5 | $\scriptstyle {\left.{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right\}\,$ | 4 Endognaths | 4 Tentacles | 1 Tentacular to 4 tentacles | 1 Tentacular bar to 4 tentacles | 1 Oculomotor supplying 4 muscles | Sup. inf. int. rectus and inf. oblique | 1 Premandibular fusion of 4 | 1 Pituitary body; fusion of 4 coxal glands |
| 2 | 6 | | 1 Ectognath | 1 Tongue | 1 Tongue nerve | 1 Tongue bar | 1 Trochlearis supplying 1 muscle | Sup. oblique | 1 Mandibular | |
The Tubular Muscles.
The only musculature innervated by the trigeminal nerve which remains for further discussion, consists of those peculiar muscles found in the velum, known by the name of striated tubular muscles. This group of muscles has already been referred to in Chapter IV., dealing with respiration and the origin of the heart.
It is a muscular group of extraordinary interest in seeking an answer to the question of vertebrate ancestry, for, like the thyroid gland, it bears all the characteristics of a survival from a prevertebrate form, which is especially well marked in Ammocœtes. I have already suggested in this chapter that the homologues of these muscles are represented in Limulus by the veno-pericardial group of muscles. I will now proceed to deal with the evidence for this suggestion.
The structure of the muscle-fibres is peculiar and very characteristic, so that wherever they occur they are easily recognized. Each fibre consists of a core of granular protoplasm, in the centre of which the nuclei are arranged in a single row. This core is surrounded by a margin of striated fibrillæ, as is seen in Fig. 122. Such a structure is characteristic of various forms of striated muscle found in various invertebrates, such as the muscle-fibre of mollusca. It is, as far as I know, found nowhere in the vertebrate kingdom, except in Ammocœtes. At transformation these muscles entirely disappear, becoming fattily degenerated and then absorbed.
Fig. 122.—A Tubular Muscle-fibre of Ammocœtes.
For all these reasons they bear the stamp of a survival from a prevertebrate form. This alone would not make this tissue of any great importance, but when in addition these muscles are found to be arranged absolutely segmentally throughout the whole of the branchial region, then this tissue becomes a clue of the highest importance.
As mentioned in Chapter IV., the segmental muscles of respiration consist of the adductor muscle and the two constrictor muscles—the ​striated constrictor and the tubular constrictor. Of these muscles, both the muscles possessing ordinary striation are attached to the branchial cartilaginous skeleton, whereas the tubular constrictors have nothing to do with the cartilaginous basket-work, but are attached ventrally in the neighbourhood of the ventral aorta.
These segmental tubular muscles are found also in the velar folds—the remains of the septum or velum which originally separated the oral from the respiratory chamber. In the branchial region they act with the other constrictors as expiratory muscles, forcing the water out of the respiratory chamber. In the living Ammocœtes, the velar folds on each side can be seen to move synchronously with the movements of respiration, contracting at each expiration; they thus close the slit by which the oral and respiratory chambers communicate, and therefore, in conjunction with the respiratory muscles, force the water of respiration to flow out through the gill-slits, as described by Schneider.
These tubular muscles thus form a dorso-ventral system of muscles essentially connected with respiration; they belong to each one of the respiratory segments, and are also found in the velum; anterior to this limit they are not to be found. What, then, are these tubular muscles in the velar folds? Miss Alcock has worked out their topography by means of serial sections, and, as already fully explained, has shown that they form exactly similar dorso-ventral groups, which belong to the two segments anterior to the purely branchial segments, i.e. to the facial or hyoid segments and the lower lip-segment of the trigeminal nerve. If the velar folds could be put back into their original position as a septum, then the hyoid or facial group of tubular muscles would take up exactly the same position as those belonging to each branchial segment.
The presence of these two so clearly segmental groups of muscles in the velum—the one belonging to the region of the trigeminal, the other to the region of the facial—is strong confirmation of my contention that this septum between the oral and respiratory chambers was caused by the fusion of the last prosomatic and the first mesosomatic appendages, represented in Limulus by the chilaria and the operculum.
Yet another clue to the meaning of these muscles is to be found in their innervation, which is very extraordinary and unexpected. Throughout the branchial region the striated muscles of each segment ​are strictly supplied by the nerve of that segment, and, as already described, each segment is as carefully mapped out in its innervation as it is in any arthropod appendage. One exception occurs to this orderly, symmetrical arrangement: a nerve arises in connection with the facial nerve, and passes tailwards throughout the whole of the branchial region, giving off a branch to each segment as it passes. This nerve (Br. prof., Fig. 123) is known by the name of the ramus branchialis profundus of the facial, and its extraordinary course has always aroused great curiosity in the minds of vertebrate anatomists. Miss Alcock, by the laborious method of following its course throughout a complete series of sections, finds that each of the segmental branches which is given off, passes into the tubular muscles of that segment (Fig. 124). The tubular muscles which belong to the velum, i.e. those belonging to the lower lip-segment and to the hyoid segments, receive their innervation from the velar or mandibular nerve, and belong, therefore, to the trigeminal, not to the facial, system.
Fig. 123.—Diagram showing the Distribution of the Facial Nerve.
Motor branches, red; sensory branches, blue.
The evidence presented by these muscles is as follows:—
In the ancestor of the vertebrate there must have existed a segmentally arranged set of dorso-ventral muscles of peculiar structure, concerned with respiration, and confined to the mesosomatic segments and to the last prosomatic segment, yet differing from the other dorso-ventral muscles of respiration in their innervation and their attachment.
Interpreting these facts with the aid of my theory of the origin of vertebrates, and remembering that the homologue of the vertebrate ventral aorta in such a palæostracan as Limulus is the longitudinal ​venous sinus, while the opercular and chilarial segments are respectively the foremost mesosomatic and the last prosomatic segments; they signify that the palæostracan ancestor must have possessed a separate set of segmental dorso-ventral muscles confined to the branchial, opercular and chilarial or metastomal segments, which, on the one hand, were respiratory in function, and on the other were attached to the longitudinal venous sinus. Further, these muscles must all have received a nerve-supply from the neuromeres belonging to the chilarial and opercular segments, an unsymmetrical arrangement of nerves, on the face of it, very unlikely to occur in an arthropod.
​Is this prophecy borne out by the examination of Limulus? In the first place, these muscles were dorso-ventral and segmental, and, referring back to Chapter VII., Lankester arranges the segmental dorso-ventral muscles in three groups: (1) The dorso-ventral somatic muscles; (2) the dorso-ventral appendage muscles; and (3) the veno-pericardial muscles. Of these the first group is represented in the vertebrate by the muscles which move the eye, the second group by the striated constrictor and adductor muscles and the muscles for the lower lip. There is, then, the possibility of the third group for this system of tubular muscles.
Looking first at the structure of these muscles as previously described, so different are they in appearance from the ordinary muscles of Limulus, that Milne-Edwards, as already stated, called them "brides transparentes," and did not recognize their muscular character, while Blanchard called them in the scorpion, "ligaments contractils."
Consider their attachment and their function. They are attached to the longitudinal sinus, according to Lankester's observation, in such a way that the muscle-fibres form a hollow cone filled with blood; when they contract they force this blood towards the gills, and thus act as accessory or branchial hearts. According to Blanchard, in the scorpion they contract synchronously with the heart; according to Carlson, in Limulus they contract with the respiratory muscles. In Ammocœtes, where the respiration is effected after the fashion of Limulus, not of Scorpio, the tubular muscles are respiratory in function.
Look at their limits. The veno-pericardial muscles in Limulus are limited by the extent of the heart, they do not extend beyond the anterior limit of the heart. In Fig. 70 (p. 176) two of these muscles are seen in front of the branchial region also attached to the longitudinal venous sinus, although in front of the gill-region. In Ammocœtes the upper limit of the tubular muscles is the group found in the velum; this most anterior group belongs to a region in front of the branchial region—that of the trigeminal.
Moreover, the supposition that the segmental tubular muscles belong throughout to the veno-pericardial group gives an adequate reason why they do not occur in front of the velum; for, as their existence is dependent upon the longitudinal collecting sinus in Limulus and Scorpio, which is represented by the ventral aorta in ​Ammocœtes, they cannot extend beyond its limits. Now, Dohrn asserts that the ventral aorta terminates in the spiracular artery, which exists only for a short time; and, in another place, speaking of this same termination of the ventral aorta, he states: "Dass je eine vorderste Arterie aus den beiden primären Aesten des Conus arteriosus hervorgeht, die erste Anlage der Thyroidea umfasst, in der Mesodermfalte des späteren Velums in die Höhe steigt um in die Aorta der betreffenden Seite einzumunden." These observations show that the vessel which in Ammocœtes represents the longitudinal collecting sinus in the Merostomata does not extend further forwards than the velum, and in consequence the representatives of the veno-pericardial muscles cannot extend into the segments anterior to the velum. One of the extraordinary characteristics of these tubular muscles which distinguishes them from other muscles, but brings them into close relationship with the veno-pericardial group, is the manner in which the bundles of muscle-fibres are always found lying freely in a blood-space; this is clearly seen in the branchial region, but most strikingly in the velum, the interior of which, apart from its muco-cartilage, is simply a large lacunar blood-space traversed by these tubular muscles.
All these reasons point to the same conclusion: the tubular muscles in Ammocœtes are the successors of the veno-pericardial system of muscles.
If this is so, then this homology ought to throw light on the extraordinary innervation of these tubular muscles by the branchialis profundus branch of the facial nerve and the velar branch of the trigeminal. We ought, in fact, to find in Limulus a nerve arising exclusively from the ganglia belonging to the chilarial and opercular segments, which, instead of being confined to those segments, traverses the whole branchial region on each side, and gives off a branch to each branchial segment; this branch should supply the veno-pericardial muscle of that side.
Patten and Redenbaugh have traced out the distribution of the peripheral nerves in Limulus, and have found that from each mesosomatic ganglion a segmental cardiac nerve arises which passes to the heart and there joins the cardiac median nerve, or rather the median heart-ganglion, for this so-called nerve is really a mass of ganglion-cells. In all the branchial segments the same plan exists, each cardiac nerve belonging to that neuromere is strictly segmental. ​Upon reaching the opercular and chilarial neuromeres an extraordinary exception is found; the cardiac nerves of these two neuromeres are fused together, run dorsally, and then form a single nerve called the pericardial nerve, which runs outside the pericardium along the whole length of the mesosomatic region, and gives off a branch to each of the cardiac nerves of the branchial neuromeres as it passes them.
This observation of Patten and Redenbaugh shows that the pericardial nerve of Limulus agrees with the very nerve postulated by the theory, as far as concerns its origin from the chilarial and opercular neuromeres, its remarkable course along the whole branchial region, and its segmental branches to each branchial segment.
At present the comparison goes no further; there is no evidence available to show what is the destination of these segmental branches of the pericardial nerve, and so far all evidence of their having any connection with the veno-pericardial muscles is wanting. Carlson, at my request, endeavoured in the living Limulus to see whether stimulation of the pericardial nerve caused contraction of the veno-pericardial muscles, but was unable to find any such effect. On the contrary, his experimental work indicated that each veno-pericardial muscle received its motor supply from the corresponding mesosomatic ganglion. This is not absolutely conclusive, for if, as Blanchard asserts in the case of the scorpion, a close connection exists between the action of these muscles and of the heart, it is highly probable that their innervation conforms to that of the heart. Now Carlson has shown that this cardiac nerve from the opercular and chilarial neuromeres is an inhibitory nerve to the heart, while the segmental cardiac nerves belonging to the branchial ganglia are the augmentor nerves of the heart.
His experiments, then, show that the motor nerves of the heart and of the veno-pericardial muscles run together in the same nerves, but he says nothing of the inhibitory nerves to the latter muscles. If they exist and if they are in accordance with those to the heart, then they ought to run in the pericardial nerve, and would naturally reach the veno-pericardial muscles by the segmental branches of the pericardial nerve.
Moreover, inhibitory nerves are, in certain cases, curiously associated with sensory fibres; so that the nerve which corresponds ​to the pericardial nerve, viz. the branchialis profundus of the facial, may be an inhibitory and sensory nerve, and not motor at all. Miss Alcock's observations are purely histological; no physiological experiments have been made.
At present, then, it does not seem to me possible to say that Carlson's experiments have disproved any connection of the pericardial nerve with the veno-pericardial muscles. We do not know what is the destination of its segmental branches; they may still supply the veno-pericardial muscles even if they do not cause them to contract; they certainly do not appear to pass directly into them, for they pass into the segmental cardiac nerves, and can only reach the muscles in conjunction with their motor nerves. Such a course would not be improbable when it is borne in mind how, in the frog, the augmentor nerves run with the inhibitory along the whole length of the vagus nerve.
Until further evidence is given both as to the function of the segmental branches of the pericardial nerve in the Limulus, and of the branchialis profundus in Ammocœtes, it is impossible, I think, to consider that the phylogenetic origin of these tubular muscles is as firmly established as is that of most of the other organs already considered. I must say, my own bias is strongly in favour of looking upon them as the last trace of the veno-pericardial system of muscles, a view which is distinctly strengthened by Carlson's statement that the latter system contracts synchronously with the respiratory movements, for undoubtedly in Ammocœtes their function is entirely respiratory. Then again, although at present there is no evidence to connect the pericardial nerve in Limulus with this veno-pericardial system of muscles, yet it is extraordinarily significant that in such animals as Limulus and Ammocœtes, in both of which the mesosomatic or respiratory region is so markedly segmental, an intrusive nerve should, in each case, extend through the whole region, giving off branches to each segment. Still more striking is it that this nerve should arise from the foremost mesosomatic and the last prosomatic neuromeres in Limulus—the opercular and chilarial segments—precisely the same neuromeres which give origin to the corresponding nerve in Ammocœtes, for according to my theory of the origin of vertebrates, the nerves which supplied the opercular and metastomal appendages have become the facial nerve and the lower lip-branch of the trigeminal nerve.
​With the formation of the vertebrate heart from the two longitudinal venous sinuses and the abolition of the dorsal invertebrate heart, the function of these tubular muscles as branchial hearts was no longer needed, and their respiratory function alone remained. The last remnant of this is seen in Ammocœtes, for the ordinary striated muscles were always more efficient for the respiratory act, and so at transformation the inferior tubular musculature was got rid of, there being no longer any need for its continued existence.
The Palæostoma, or Old Mouth.
The arrangement of the oral chamber in Ammocœtes is peculiar among vertebrates, and, upon my theory, is explicable by its comparison with the accessory oral chamber which apparently existed in Eurypterus. According to this explanation, the lower lip of the original vertebrate mouth was formed by the coalescence of the most posterior pair of the prosomatic appendages—the chilaria; from which it follows that the vertebrate mouth was not the original mouth, but a new structure due to such a formation of the lower lip.
It is very suggestive that the direct following out of the original working hypothesis should lead to this conclusion, for it is universally agreed by all morphologists that the present mouth is a new formation, and Dohrn has argued strongly in favour of the mouth being formed by the coalescence of a pair of gill-slits. Interpret this in the language of my theory, and immediately we see, as already explained, gill-slits must mean in this region the spaces between appendages which did not carry gills; the mouth, therefore, was formed by the coalescence of a pair of appendages to form a lower lip just as I have pointed out.
Where, then, must we look for the palæostoma, or original mouth? Clearly, as already suggested, it was situated at the base of the olfactory passage, and the olfactory passage or nasal tube of Ammocœtes was originally the tube of the hypophysis, so that the following out of the theory points directly to the tube of the hypophysis as the place where the palæostoma must be looked for.
This conclusion is not only not at variance with the opinions of morphologists, but gives a straightforward, simple explanation why the palæostoma was situated in the very place where they are most inclined to locate it. Thus, if we trace the history of the question, ​we see that Dohrn's original view of the comparison of the vertebrate and the annelid led him to the conception that the vertebrate mouth was formed by the coalescence of a pair of gill-slits, and that the original mouth was situated somewhere on the dorsal surface and opened into the gut by way of the infundibulum and the tube of the hypophysis. This, also, was Cunningham's view as far as the tube of the hypophysis was concerned. Beard, in 1888, holding the view that the vertebrates were derived from annelids which had lost their supra-œsophageal ganglia, and that, therefore, there was no question of an œsophageal tube piercing the central nervous system of the vertebrate, explained the close connection of the infundibulum with the hypophysis by the comparison of the tube of the hypophysis with the annelidan mouth, so that the infundibular or so-called nervous portion was a special nervous innervation for the original throat, just as Kleinenberg had shown to be the case in many annelids. Beard therefore called this opening of the hypophysial tube the old mouth, or palæostoma. Recently, in 1893, Kupffer has also put forward the view that the hypophysial opening is the palæostoma. basing this view largely upon his observations on Ammocœtes and Acipenser.
Fig. 125.—Diagram to show the Meeting of the Four Tubes in such a Vertebrate as the Lamprey.
As is seen in Fig. 125, the position of this palæostoma is a very suggestive one. At this single point in Ammocœtes, four separate tubes terminate; here is the end of the notochordal tube, the termination of the infundibulum, the blind end of the nasal tube or tube ​of the hypophysis, and the pre-oral elongation of the alimentary canal.
It is perfectly simple and easy for the olfactory tube to open into any one of the other three. By opening into the infundibulum it reproduces the condition of affairs seen in the scorpion; by opening into the gut it produces the actual condition of things seen in Myxine and other vertebrates; by opening into the notochordal tube it would produce a transitional condition between the other two.
The view held by Kupffer is that this nasal tube (tube of the hypophysis) opened into the anterior diverticulum of the vertebrate gut, and was for this reason the original mouth-tube; then a new mouth was formed, and this connection was closed, being subsequently reopened as in Myxine. My view is that this tube originally opened into the infundibulum, in other words, into the original gut of the palæostracan ancestor, and was for this reason the original mouth-tube, in the same sense as the olfactory passage of the scorpion may be, and often is, called the mouth-tube. When, with the breaking through of the septum between the oral and respiratory chambers, the external opening of the oral chamber became a new mouth, the old mouth was closed but the olfactory tube still remained, owing to the importance of the sense of smell. Subsequently, as in Myxine and the higher vertebrates, it opened into the pharynx, and so formed the nose of the higher vertebrates.
It is not, to my mind, at all improbable that during the transition stage, between its connection with the old alimentary canal, as in Eurypterus or the scorpions, and its blind ending, as in Ammocœtes, the nasal tube opened into the tube of the notochord. This question will be discussed later on when the probable significance of the notochord is considered.
The Pituitary Gland.
Turning back to the comparison of Fig. 106, B, and Fig. 106, C, which represent respectively an imaginary sagittal section through an Eurypterus-like animal and through Ammocœtes at a larval stage, all the points for comparison mentioned on p. 244 have now been discussed with the exception of the suggested homology between the coxal glands of the one animal and the pituitary body of the other.
​This latter gland undoubtedly arises posteriorly to the hypophysial tube, or Rathke's pouch (as it is sometimes called), and, as already mentioned, is supposed by Kupffer to be formed from the posterior wall of this pouch. More recently, as pointed out in Haller's paper, Nusbaum, who has investigated this matter, finds that the glandular hypophysis is not formed from the walls of Rathke's pouch, but from the tissue of the rudimentary connection or stalk between the two premandibular cavities, which becomes closely connected with the posterior wall of Rathke's pouch, and becoming cut off from the rest of the premandibular cavity on each side, becomes permanently a part of the 'Hypophysis Anlage.'
The importance of Nusbaum's investigation consists in this, that he derives the glandular hypophysis from the connecting stalk between the two premandibular cavities, and therefore from the walls of the ventral continuation of this cavity on each side.
This may be expressed as follows:—
The cœlomic cavity, known as the premandibular cavity, divides into a dorsal and a ventral part; the walls of the dorsal part give origin to the somatic muscles belonging to the oculomotor nerve, while the walls of the ventral part on each side form the connecting stalk between the two cavities, and give origin to the glandular hypophysis.
Now, as already pointed out, the premandibular cavity is homologous with the 2nd prosomatic cœlomic cavity of Limulus, and this 2nd prosomatic cœlomic cavity divides, according to Kishinouye, into a dorsal and a ventral part; and, further, the walls of this ventral part form the coxal gland. Both in the vertebrate, then, and in Limulus, we find a marked glandular tissue in a corresponding position, and the conclusion is forced upon us that the glandular hypophysis was originally the coxal gland of the invertebrate ancestor. As in all other cases already considered, when the facts of topographical anatomy, of morphology and of embryology, all combine to the same conclusion as to the derivation of the vertebrate organ from that of the invertebrate, then there must be also a structural similarity between the two. What, then, is the nature of the coxal gland in the scorpions and Limulus? Lankester's paper gives us full information on this point as far as the scorpion and Limulus are concerned, and he shows that the coxal gland of Limulus differs markedly from that of Scorpio in the size of the cells and in the ​arrangement of the tubes. In Fig. 126, A, I give a picture of a piece of the coxal gland of Limulus taken from Lankester's paper.
Turning now to the vertebrate, Bela Haller's paper gives us a number of pictures of the glandular hypophysis from various vertebrates, and he especially points out the tubular nature of the gland and its solidification in the course of development in some cases. In Fig. 126, B, I give his picture of the gland in Ammocœtes.
The striking likeness between Haller's picture and Lankester's picture is apparent on the face of it, and shows clearly that the histological structure of the glands in the two cases confirms the deductions drawn from their anatomical and morphological positions.
Fig. 126.—A, Section of Coxal Gland of Limulus (from Lankester); B, Section of Pituitary Body of Ammocœtes (from Bela Haller).
n.a., termination of nasal passage.
The sequence of events which gave rise to the pituitary body of the vertebrate was in all probability somewhat as follows:—
Starting with the excretory glands of the Phyllopoda, known as shell-glands, which existed almost certainly in the phyllopod Trilobite, we pass to the coxal gland of the Merostomata. Judging from Limulus, these were coextensive with the coxæ of the 2nd, 3rd, 4th, and 5th locomotor appendages. When these appendages became reduced in size and purely tactile they were compressed and concentrated round the mouth region, forming the endognaths of the Merostomata; as a necessary consequence of the concentration of the coxæ of the endognaths, the coxal gland also became concentrated, ​and took up a situation close against the pharynx, as represented in Fig. 106, B. When, then, the old mouth closed, and the pharynx became the saccus vasculosus, the coxal gland remained in close contact with the saccus vasculosus, and became the pituitary body, thus giving the reason why there is always so close a connection between the pituitary body and the infundibular region.
Whatever was the condition of the digestive tracts at the transition stage between the arthropod and the vertebrate, the original mouth-opening at the base of the olfactory tube was ultimately closed. The method of its closure was exceedingly simple and evident. The membranous cranium was in process of formation by the extension of the plastron laterally and dorsally; a slight growth of the same tissue in the region of the mouth would suffice to close it and thus separate the infundibulum from the olfactory tube. As evidence that such was the method of closure, it is instructive to see how in Ammocœtes the glandular tissue of the pituitary body is embedded in and mixed up with the tissue of this cranial wall; how the termination of the nasal tube is embedded in this same thickened mass of the cranial wall—how, in fact, both coxal gland and olfactory tube have become involved in the growth of the tissue of the plastron, by means of which the mouth was closed.
I have now passed in review the nature of the evidence which justifies a comparison between the segments supplied by the cranial nerves of the vertebrate and the prosomatic and mesosomatic segments of the palæostracan. For the convenience of my readers I have put these conclusions into tabular form (see p. 323), for all the segments as far as that supplied by the glossopharyngeal nerves. In both vertebrate and invertebrate this is a fixed position, for in the former, however variable may be the number of branchial segments which the vagus supplies, the second branchial segment is always supplied by a separate nerve, the glossopharyngeal, and in the latter, though the number of segments bearing branchiæ varies, the minimum number of such segments (as seen in the Pedipalpi) is never less than two.
Table of Comparison of Corresponding Segments in the Eurypterids and in Ammocœtes (i.e. in Cephalaspids).
| S u p r a - o e s o p h a g e a l | Median Eyes. | Median Eyes. | Median Eyes. | Median Eyes. | Median Eyes. | Pineal Eyes. | Pineal Eyes. | Pineal Eyes. | Pineal Eyes. | Pineal Nerve. | Pineal Nerve. | S u p r a - i n f u n d i b u l a r |
| S u p r a - o e s o p h a g e a l | Lateral Eyes. | Lateral Eyes. | Lateral Eyes. | Lateral Eyes. | Lateral Eyes. | Lateral Eyes. | Lateral Eyes. | Lateral Eyes. | Lateral Eyes. | II | II | | S u p r a - i n f u n d i b u l a r |
| S u p r a - o e s o p h a g e a l | Camerostome. | Camerostome. | Camerostome. | Camerostome. | Camerostome. | Olfactory Organ. | Olfactory Organ. | Olfactory Organ. | Olfactory Organ. | I | I | | S u p r a - i n f u n d i b u l a r |
| | Invertebrate (Limulus or Eurypterid). | Invertebrate (Limulus or Eurypterid). | Invertebrate (Limulus or Eurypterid). | Invertebrate (Limulus or Eurypterid). | Invertebrate (Limulus or Eurypterid). | Vertebrate (Ammocœtes or Cephalaspid). | Vertebrate (Ammocœtes or Cephalaspid). | Vertebrate (Ammocœtes or Cephalaspid). | Vertebrate (Ammocœtes or Cephalaspid). | Vertebrate (Ammocœtes or Cephalaspid). | Vertebrate (Ammocœtes or Cephalaspid). | Vertebrate (Ammocœtes or Cephalaspid). |
| I n f r a - o e s o p h a g e a l | P r o s o m a t i c | Segments. | Appendages. | Appendages. | Cœlomic cavities | Appendages. | Splanchnic Nerves. | Somatic Segmental Muscles. | Somatic Nerves. | Cœlomic Cavities. | V. Wijhe's Segments. | I n f r a - i n f u n d i b u l a r |
| I n f r a - o e s o p h a g e a l | P r o s o m a t i c | Segments. | Limulus. | Eurypterid. | Cœlomic cavities | Appendages. | Splanchnic Nerves. | Somatic Segmental Muscles. | Somatic Nerves. | Cœlomic Cavities. | V. Wijhe's Segments. | I n f r a - i n f u n d i b u l a r |
| I n f r a - o e s o p h a g e a l | P r o s o m a t i c | 1 | Cheliceræ | Cheliceræ | 1 | — | — | — | — | Anterior. | — | I n f r a - i n f u n d i b u l a r |
| I n f r a - o e s o p h a g e a l | P r o s o m a t i c | 2 | 1st Locomotor | $\scriptstyle {\left.{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right\}\,$ 4 Endognaths | 2 Ventral part forms coxal gland. | 4 tentacles and upper lip. | V Tentacular and upper lip nerve. | Muscles supplied by oculomotor nerve. | III | Premandibular. Ventral part forms pituitary body. | 1 | I n f r a - i n f u n d i b u l a r |
| I n f r a - o e s o p h a g e a l | P r o s o m a t i c | 3 | 2nd Loct" | $\scriptstyle {\left.{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right\}\,$ 4 Endognaths | 2 Ventral part forms coxal gland. | 4 tentacles and upper lip. | V Tentacular and upper lip nerve. | Muscles supplied by oculomotor nerve. | III | Premandibular. Ventral part forms pituitary body. | 1 | I n f r a - i n f u n d i b u l a r |
| I n f r a - o e s o p h a g e a l | P r o s o m a t i c | 4 | 3rd Loco" | $\scriptstyle {\left.{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right\}\,$ 4 Endognaths | 2 Ventral part forms coxal gland. | 4 tentacles and upper lip. | V Tentacular and upper lip nerve. | Muscles supplied by oculomotor nerve. | III | Premandibular. Ventral part forms pituitary body. | 1 | I n f r a - i n f u n d i b u l a r |
| I n f r a - o e s o p h a g e a l | P r o s o m a t i c | 5 | 4th Loco" | $\scriptstyle {\left.{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right\}\,$ 4 Endognaths | 2 Ventral part forms coxal gland. | 4 tentacles and upper lip. | V Tentacular and upper lip nerve. | Muscles supplied by oculomotor nerve. | III | Premandibular. Ventral part forms pituitary body. | 1 | I n f r a - i n f u n d i b u l a r |
| I n f r a - o e s o p h a g e a l | P r o s o m a t i c | 6 | 5th Loco" | Ectognath | 3 | Tongue | V Tongue nerve | Sup. oblique | IV | Mandibular | $\scriptstyle {\left.{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right\}\,$ 2 | I n f r a - i n f u n d i b u l a r |
| I n f r a - o e s o p h a g e a l | P r o s o m a t i c | 7 | Chilaria | Metastoma | 4 | Lower lip | V Lower lip nerve | | | Mandibular | $\scriptstyle {\left.{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right\}\,$ 2 | I n f r a - i n f u n d i b u l a r |
| I n f r a - o e s o p h a g e a l | M e s o s o m a t i c | 8 | Operculum | Genital $\scriptstyle {\left.{\begin{matrix}\ \\\\\ \ \end{matrix}}\right\}\,$ Operculum Branchial | 5 | Thyroid | $\scriptstyle {\left.{\begin{matrix}\ \\\\\ \ \end{matrix}}\right\}\,$ VII | Ext. rectus Retract oculi | VI | Hyoid$_1$ | 3 | I n f r a - i n f u n d i b u l a r |
| I n f r a - o e s o p h a g e a l | M e s o s o m a t i c | 9 | 1st Branchial | Genital $\scriptstyle {\left.{\begin{matrix}\ \\\\\ \ \end{matrix}}\right\}\,$ Operculum Branchial | 6 | Hyoid or 1st Branchial | $\scriptstyle {\left.{\begin{matrix}\ \\\\\ \ \end{matrix}}\right\}\,$ VII | Ext. rectus Retract oculi | VI | Hyoid$_2$ | 4 | I n f r a - i n f u n d i b u l a r |
| I n f r a - o e s o p h a g e a l | M e s o s o m a t i c | 100 | 2nd Bra" | 2nd Branchial | 7 | Branchial | IX | — | — | 2nd Branchial | 5 | I n f r a - i n f u n d i b u l a r |
Summary.
The general consideration of the evidence of the number of segments, and their nature in the pro-otic region of the vertebrate, as given in the last chapter, is not incompatible with the view that the trigeminal nerve originally supplied seven appendages, which appendages did not carry branchiæ, but were originally used for purposes of locomotion as well as of mastication.
Such appendages clearly no longer exist in the higher vertebrates, the muscles of mastication only remaining; but in the earliest fish-forms they must have existed, as, indeed, is seen in Ptericthys and Bothriolepis. Judging from all the previous evidence some signs of their existence may reasonably be expected still to remain in Ammocœtes. Such is indeed the case.
In the adult Petromyzon the trigeminal nerve innervates specially a massive suctorial apparatus, by means of which it holds on to other fishes, or to stones in the bottom of the stream. There is here no apparent sign of appendages. Very great, however, is the difference in the oral chamber of Ammocœtes; here there is no sign of any suctorial apparatus, but instead, a system of tentacles, together with the remains of the septum or velum, which originally closed off the oral from the respiratory chamber. These tentacles are the last remnants of the original foremost prosomatic appendages of the palæostracan ancestor. Like the lateral eyes they do not develop until the transformation comes, but during the whole larval condition their musculature remains in an embryonic condition, and then from these embryonic muscles the whole massive musculature of the suctorial apparatus develops; a sucking apparatus derived from the modification of appendages, as so frequently occurs in the arthropods.
The study of Ammocœtes indicates that the velum and lower lip correspond to the metastoma of the Eurypterid, i.e. the chilaria of Limulus, while the large ventral pair of tentacles, called the tongue, correspond to the ectognaths of the Eurypterids, and probably to the oar-like appendages of Ptericthys and Bothriolepis. From these two splanchnic segments the suctorial apparatus in the main arises; the motor supply of these two segments forms the mass of the trigeminal nerve-supply, and the nerves supplying them, the velar nerve and the tongue-nerve, are markedly separate from the rest of the trigeminal nerve.
The rest of the tentacles present much less the sign of independent segments. In their nerves, their muco-cartilaginous skeleton, and their rudimentary muscles, they indicate a concentration and amalgamation, such as might be expected from the concentrated endognaths. The continuation of the dwindling process, already initiated in the Eurypterid, would easily result in the tentacles of Ammocœtes.
The nasal tube of Ammocœtes, which originates in the hypophysial tube, corresponds absolutely in position and in its original structure, to the olfactory tube of a scorpion-like animal. From this homology two conclusions of importance follow: (1) the old mouth, or palæostoma, of the vertebrate was situated at the end of this tube, therefore, at the termination of the infundibulum; (2) the upper lip, which by its growth, brings the olfactory tube from a ventral to a dorsal position, was originally formed by the foremost sternites or endostoma, or else by the sterno-coxal processes of the second pair of prosomatic appendages of the palæostracan ancestor.
In strict accordance with the rest of the comparisons made in this region, the pituitary body shows by similarity of structure, as well as of position, that it arose from the coxal glands, which were situated at the base of the four endognaths.
​One after another, when once the clue has been found, all these mysterious organs of the vertebrate, such as the pituitary and thyroid glands, fall harmoniously into their place as the remnants of corresponding important organs in the palæostraca.
Yet another clue is afforded by the tubular muscles of Ammocœtes, that strange set of non-vertebrate striated muscles, which are so markedly arranged in a segmental manner, which disappear at transformation, and are never found in any of the higher vertebrates, for the limits of their distribution correspond to the veno-pericardial muscles of Limulus.
Their nerve-supply in Ammocœtes is most extraordinary; for, although they are segmentally arranged throughout the whole respiratory region, which is segmentally supplied by the VIIth, IXth, and Xth nerves, and are found in front of this region only in one segment, that of the lower lip, which is supplied by the velar branch of the Vth nerve, yet they are not supplied segmentally, but only by the velar nerve and a branch of the VIIth, the ramus branchialis profundus. This latter nerve extends throughout the respiratory region, and gives off segmental branches to supply these muscles.
It is also a curious coincidence that in such a markedly segmented animal as Limulus, a nerve—the pericardial nerve—which arises from the nerves of the chilarial and opercular segments, should pass along the whole respiratory region and give off branches to each mesosomatic segment. It is strange, to say the least of it, that the chilarial or metastomal and the opercular segments of Limulus should, on the theory advocated in this book, correspond to the lower lip and hyoid segments of the vertebrate. At present the homology suggested is not complete, for there is no evidence as yet that the veno-pericardial muscles have anything to do with the pericardial nerve.