# An Introduction to the Study of Fishes/Chapter 2

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CHAPTER II.
TOPOGRAPHICAL DESCRIPTION OF THE EXTERNAL PARTS OF FISHES.
In the body of a fish four parts are distinguished: the head, trunk, tail, and the fins; the boundary between the first and second being generally indicated by the gill-opening, and that between the second and third by the vent. Form of the body. The form of the body and the relative proportions of those principal parts are subject to much variation, such as is not found in any other class of Vertebrates. In fishes which are endowed with the power of steady and more or less rapid locomotion, a deviation from that form of body, which we observe in a perch, carp, or mackerel, is never excessive. The body forms a simple, equally-formed wedge, compressed or slightly rounded, well fitted for cleaving the water. In fishes which are in the habit of moving on the bottom, the whole body, or at least the head, is vertically depressed and flattened; the head may be so enormously enlarged that the trunk and tail appear merely as an appendage. In one family of fishes, the Pleuronectidæ or Flat-fishes, the body is compressed into a thin disk; they swim and move on one side only, which remains constantly directed towards the bottom, a peculiarity by which the symmetry of all parts of the body has been affected. A lateral compression of the body, in conjunction with a lengthening of the vertical and a shortening of the longitudinal axis, we find in fishes moving comparatively slowly through ​the water, and able to remain (as it were) suspended in it. This deviation from the typical form may proceed so far that the vertical axis greatly exceeds the longitudinal in length; generally all the parts of the body participate in this form, but in one kind of fish (the Sun-fish or Orthagoriscus) it is chiefly the tail which has been shortened, and reduced so much as to present the appearance of being cut off. An excessive lengthening of the longitudinal axis, with a shortening of the vertical, occurs in Eels and eel-like fishes, and in the so-called Band-fishes. They are bottom-fish, capable of insinuating themselves into narrow crevices and holes. The form of the body of these long fish is either cylindrical, snake-like, as in the Eels and many Codfishes, or strongly compressed as in the Band-fishes (Trichiurus, Regalecus, etc.) It is chiefly the tail which is lengthened, but frequently the head and trunk participate more or less in this form. Every possible variation occurs between these and other principal types of form. The old ichthyologists, even down to Linneeus, depended in great measure on them for classification; but although often the same form of body obtains in the same group of fishes, similarity of form by no means indicates natural affinity; it only indicates similitude of habits and mode of life.
Eye. The external parts of the Head.—The Eye divides the head into the ante-orbital and post-orbital portion. In most fishes, especially in those with a compressed head, it is situated on the side and in the anterior half of the length of the head; in many, chiefly those with a depressed head, it is directed upwards, and sometimes situated quite at the upper side; in very few, the eyes look obliquely downwards. In the Flat-fishes both eyes are on the same side of the head, either the right or the left, always on that which is directed towards the light, and coloured.
Fishes in general, compared with other Vertebrata, have large eyes. Sometimes these organs are enormously enlarged, ​their great size indicating that the fish is either nocturnal, or lives at a depth to which only a part of the sun's rays penetrate. On the other hand, small eyes occur in fishes inhabiting muddy places, or great depths to which scarcely any light descends, or in fishes in which the want of an organ of sight is compensated by the development of other organs of sense. In a few fishes, more particularly in those inhabiting caves or the greatest depths of the ocean, the eyes have become quite rudimentary and hidden under the skin.
Snout. In the ante-orbital portion of the head, or the Snout, are situated the mouth and the nostrils.
Mouth. The Mouth is formed by the intermaxillary and maxillary bones, or by the intermaxillary only in the upper jaw, and by the mandibulary bone in the lower. These bones are either bare or covered by integument, to which frequently labial folds or lips are added. As regards form, the mouth offers as many variations as the body itself, in accordance with the nature of the food, and the mode of feeding. It may be narrow, or extremely wide and cleft to nearly the hind margin of the head; it may be semi-elliptical, semicircular, or straight in a transverse line; it may be quite in front of the snout (anterior), or at its upper surface (superior), or at its lower (inferior), or extending along each side (lateral); sometimes it is sub-circular, organised for sucking. The jaws of some fishes are modified into a special weapon of attack (Sword-fish, Saw-fish); in fact, throughout the whole class of fishes the jaws are the only organ specialised for the purpose of attacking; weapons on other parts of the body are purely defensive.
Both jaws may be provided with skinny appendages, barbels, which, if developed and movable, are sensitive organs of touch.
Nostrils. In the majority of fishes the Nostrils are a double opening on each side of the upper surface of the snout; the openings ​of each side being more or less close together. They lead into a shallow groove; and only in one family (the Myxinoids) perforate the palate. In this family, as well as in the Lampreys, the nasal aperture is single. In many Eels the openings are lateral, the lower perforating the upper lip. In the Sharks and Rays (Fig. 1, p. 34) they are at the lower surface of the snout, and more or less confluent; and, finally, in the Dipnoi and other Ganoids, one at least is within the labial boundary of the mouth.
The space across the forehead, between the orbits, is called the inter-orbital space; that below the orbit, the infra-orbital or sub-orbital region.
Gill-cover. In the post-orbital part of the head there are distinguished, at least in most Teleosteous Fishes and many Ganoids, (Fig. 24) the præoperculum, a sub-semicircular bone, generally with a free and often serrated or variously-armed margin; the operculum, forming the posterior margin of the gill-opening, and the suboperculum and inter-operculum along its inferior margin. All these bones, collectively called opercles, form the gill-cover, a thin bony lamella covering the cavity containing the gills. Sometimes they are covered with so thin a membrane that the single bones may be readily distinguished; sometimes they are hidden under a thick integument. In some cases the inter-operculum is rudimentary or entirely absent (Siluroids).
Gill-opening. The Gill-opening is a foramen, or a slit behind or below the head, by which the water which has been taken up through the mouth for the purpose of breathing is again expelled. This slit may extend from the upper end of the operculum all round the side of the head to the symphysis of the lower jaw; or it may be shortened and finally reduced to a small opening on any part of the margin of the gill-cover. Sometimes (Symbranchus) the two openings, thus reduced, coalesce, and form what externally appears as a single opening only. The margin of the gill-cover is provided with a cutaneous fringe, ​in order to more effectually close the gill-opening; and this fringe is supported by one or several or many bony rays, the branchiostegals. The space on the chest between the two rami of the lower jaw and between the gill-openings is called the isthmus.
The Sharks and Rays differ from the Teleosteous and Ganoid fishes in having five branchial slits (six or seven in Hexanchus and Heptanchus), which are lateral in the Sharks, and at the lower surface of the head in the Rays (Fig. 1, p. 34). In Myxine only the gill-opening is at a great distance from the head; it is either single in this family (Cyclostomi), or there are six and more on each side (Fig. 2).
Fig. 2.—Head of Mordacia mordax, showing the single nostril, and seven branchial openings.
In the Trunk are distinguished the back, the sides, and the abdomen. It gradually passes in all fishes into the Tail; Tail. the termination of the abdominal cavity and the commencement of the tail being generally indicated by the position of the vent. The exceptions are numerous: not only certain abdominal organs, like the sexual, may extend to between the muscles of the tail, but the intestinal tract itself may pass far backwards, or, singularly, it may be reflected forwards, so that the position of the vent may be either close to the extremity of the tail or to the foremost part of the trunk.
In many fishes the greater part of the tail is surrounded by the fins, leaving only a small portion (between dorsal, caudal, and anal fins) finless; this part is called the free portion or the peduncle of the tail.
​Fins. The Fins are divided into vertical or unpaired, and into horizontal or paired fins. Any of them may be present or absent; and their position, number, and form are most important guides in determining the affinities of fishes.
The vertical fins are situated in the median dorsal line, from the head to the extremity of the tail, and in the ventral line of the tail. In fishes in which they are least developed or most embryonic, the vertical fin appears as a simple fold of the skin surrounding the extremity of the tail. In its further progress of development in the series of fishes, it gradually extends more forwards, and may reach even the head and vent. Even in this embryonic condition the fin is generally supported by fine rays, which are the continuations of, or articulated to, other stronger rays supported by the processes or apophyses of the vertebral column. Fig. 3. 1. Simple ray. 2. Spine. 3. Simple articulated ray (soft). 4. Branched ray (soft). This form of the vertical fin is very common, for instance in the Eels, many Gadoid, Blennioid and Ganoid fishes in which, besides, the rays have ceased to be simple rods, showing more or less numerous joints (simple articulated rays; Fig. 3). Branched rays are dichotomically split, the joints increasing in number towards the extremity.
The continuity of the vertical fin, however, is interrupted in the majority of fishes; and three fins then are distinguished: one in the dorsal line—the dorsal fin; one in the ventral line behind the anus—the anal fin; and one confined to the extremity of the tail—the caudal fin.
The caudal fin is rarely symmetrical, so that its upper half would be equal to its lower; the greatest degree of asymmetry obtains in fishes with heterocercal termination of the vertebral column (see subsequently, Figs. 31, 41). In fishes in ​which it is nearly symmetrical it is frequently prolonged into an upper and lower lobe, its hind margin being concave or more or less deeply excised; in others the hind margin is rounded, and when the middle rays greatly exceed in length the outer ones the fin assumes a pointed form.
Many and systematically important differences are observed in the dorsal fin, which is either spiny-rayed (spinous) (Acanthopterygian), or soft-rayed (Malacopterygian). In the former, a smaller or greater number of the rays are simple and without transverse joints; they may be flexible, or so much osseous matter is deposited in them that they appear hard and truly spinous (Fig. 3); these spines form always the anterior portion of the fin, which is detached from, or continuous
Fig. 4.—Labrax lupus (Bass), an Acanthopterygian with anterior spinous, and posterior soft dorsal fin.
with, the remaining jointed rays. The spines can be erected or depressed at the will of the fish; if in the depressed position the spines cover one another completely, their points lying in the same line, the fish is called homacanth; but if the spines are asymmetrical, alternately broader on one side than on the other, the fish is called heteracanth. The spinous division, as well as the one consisting of jointed rays, may again be subdivided. In the Malacopterygian type all the rays remain jointed; indeed, sometimes the foremost ray, with its preceding short supports, is likewise ossified, and a hard spine, but the articulations can nearly always be distinctly ​traced. Sometimes the dorsal fin of Malacopterygian fishes is very long, extending from the head to the end of the tail, sometimes it is reduced to a few rays only, and in a few cases it is
Fig. 5.—Saurus undosquamis, a Malacopterygian with anterior soft dorsal, and additional adipose fin.
entirely absent. In addition to the rayed dorsal fin, many Malacopterygian fishes (as the Salmonoids, many Siluroids, Scopeloids, etc.) have another of greater or lesser extent, without any rays; and as always fat is deposited within this fold, it is called a fatty fin (pinna adiposa).
The anal fin is built on the same plan as the dorsal, and may be single or plural, long or short, or entirely absent; in Acanthopterygians its foremost rays are frequently simple and spinous.
The horizontal or paired fins consist of two pairs: the pectorals and ventrals.
The pectoral fins (with their osseous supports) are the homologues of the anterior limbs of the higher Vertebrata. They are always inserted immediately behind the gill-opening; either symmetrical with a rounded posterior margin, or asymmetrical, with the upper rays longest and strongest; in Malacopterygians with a dorsal spine the upper pectoral ray is frequently developed into a similar defensive weapon.
The ventral fins are the homologues of the hind-limbs, and inserted on the abdominal surface, either behind the pectorals (Pisces s. Pinnæ abdominales), or below them (Pisces s. Pinnæ thoracicæ), or in advance of them (Pisces s. Pinnæ jugulares). They are generally narrow, composed of a small number of ​rays, the outer of which is frequently osseous. In some small groups of fishes, like the Gobies, the fins coalesce and form a suctorial disk.
Fig. 6.—Salmo salar (Salmon), with abdominal ventral fins.
Fig. 7.—Mullus barbatus (Red Mullet), with thoracic ventral fins.
Fig. 8.—Burbot (Lota vulgaris), with jugular ventral fins.
For the definition of the smaller systematic groups, and the determination of species, the numbers of the spines and rays are generally of the greatest importance. This holds good, especially for the ventral rays, by the number of which ​the Acanthopterygian affinities of a fish can nearly always be determined. The numbers of the dorsal and anal rays generally correspond to the number of vertebræ in a certain portion of the spine, and are therefore constant specific, generic, or even family characters; but when their number is very great, a proportionally wide margin must be allowed for variation, and the taxinomic value of this character becomes uncertain. The numbers of the pectoral and caudal rays are rarely of any account.
Function of the Fins. The fins are organs of motion; but it is chiefly the tail and the caudal fin by which the fish impels itself forward. To execute energetic locomotion the tail and caudal fin are strongly bent, with rapidity, alternately towards the right and left; whilst a gentle motion forwards is effected by a simply undulating action of the caudal fin, the lobes of which act like the blades of a screw. Retrograde motions can be made by fish in an imperfect manner only, by forward-strokes of the pectoral fins. When the fish wants to turn towards the left, he gives a stroke of the tail towards the right, the right pectoral acting sunultaneously, whilst the left remains adpressed to the body. Thus the pectoral fins assist in the progressive motions of the fish, but rather directing its course than acting as powerful propellers. The chief function of the paired fins is to maintain the balance of the fish in the water, which is always the most unsteady where there is no weight to sink it: when the pectoral of one side, or the pectoral and ventral of the same side are removed, the fish loses its balance and falls on the side opposite; when both pectorals are removed, the fish's head sinks; on removal of the dorsal and anal fins the motion of the fish assumes a zig-zag course. A fish deprived of all fins, as well as a dead fish, floats with the belly upwards, the back being the heavier part of the body.
In numerous groups of fishes which live in mud, or are enabled to pass a longer or shorter time in soil ​periodically dried and hardened during the hot season, forms occur entirely devoid of, or with only rudimentary, ventral fins (Cyprinodon, Ophiocephalidæ, Galaxiidæ, Siluridæ). The chief function of these fins being to balance the body of the fish whilst swimming, it is evident that in fishes moving during a great part of their life over swampy ground, or through more or less consistent mud, this function of the ventral fins ceases, and that nature can readily dispense with these organs altogether.
In certain fishes the shape and function of the fins are considerably modified: thus, in the Rays, locomotion is almost entirely effected and regulated by the broad and expanded pectoral fins acting with an undulatory motion of their margins, similar to the undulations of the long vertical fins of the Flat-fishes; in many Blennies the ventral fins are adapted for walking on the sea-bottom; in some Gobioids (Periophthalmus), Trigloids, Scorpænioids, and Pediculati, the pectoral fins are perfect organs of walking; Fig. 9.—Ventrals of Gobius. in the Gobies, Cyclopteri, and Discoboli the ventral fins are transformed into an adhesive disk, and finally in the Flying-fish, in which the pectorals act as a parachute. In the Eels and other snake-like fishes, the swimming as well as the gliding motions are effected by several curvatures of the body, alternate towards the right and left, resembling the locomotion of Snakes. In the Syngnathi (Pipe-fishes) and Hippocampi, whose body admits of but a slight degree of lateral curvature, and whose caudal fin is generally small, if present at all, locomotion is very limited, and almost wholly dependent on the action of the dorsal fin, which consists of a rapid undulating movement.
Skin and Scales. The skin of fishes is either covered with scales, or naked, or provided with more or less numerous scutes of various forms and sizes. Some parts, like the head and fins, are more ​frequently naked than scaly. All fishes provided with electric organs, the majority of Eels, and the Lampreys, are naked. Scales of fishes are very different from those of Reptiles; the latter being merely folds of the cutis, whilst the scales of fishes are distinct horny elements, developed in grooves or pockets of the skin, like hairs, nails, or feathers. Fig. 10.—Cycloid scale of Gadopsis marmoratus (magn.) Very small or rudimentary scales are extremely thin, homogeneous in structure, and more or less imbedded in the skin, and do not cover each other. When more developed, they are imbricated (arranged in the manner of tiles), with the posterior part extruded and free, the surface of the anterior portion being usually covered by the skin to a greater or less extent. On their surface (Figs. 10 and 11) may be observed a very fine striation concentric and parallel to the margin, and coarser striæ radiating from a central point Fig. 11.—Cycloid scale of Scopelus resplendens (magn.) towards the hind margin. Scales without a covering of enamel, with an entire (not denticulated) posterior margin, and with a concentric striation, are called Cycloid scales. Ctenoid scales (Figs. 12-15) are generally thicker, and provided with spinous teeth on the posterior edges of the layers of which the scale consists. In some species only the layer nearest to the margin is provided with denticulations (Fig. 14). Scales, the free surface of which is spiny, and which have no denticulation on the margin, have been termed Sparoid scales; but their distinction from ctenoid scales is by no means sharp, and there are even intermediate ​forms between the cycloid and ctenoid types. Both kiads of scales may occur not only in species of the same genus of fishes, but in the same fish.
| Study of Fishes-Fig 12 | Study of Fishes-Fig 13 |
| Fig. 12.—Ctenoid scale of Scatophagus multifasciatus (magn.) | Fig. 13.—Ctenoid scale of Platycephalus cirrhonasus (magn.) |
| Study of Fishes-Fig 14 | Study of Fishes-Fig 15 |
| Fig. 14.—Ctenoid scale of Gobius ommaturus (magn.) | Fig. 15.—Ctenoid scale of Lethrinus (magn.) |
Fig. 16.—Ganoid Scales. Ganoid scales are hard and bony, covered with a layer of enamel; they are generally rhombic or quadrangular, rarely rounded and imbricate; and arranged in oblique rows, those of one row being linked together by an articulary process. This type of scales, common in fossil Ganoid fishes, occurs among recent fishes in Lepidosteus and Polypterus only.
​Finally, in Sharks, the Balistidæ, and others, true scales are absent and replaced by the ossified papillæ of the cutis, which give the surface the appearance of fine-grained chagreen.
Fig. 17.—Dermal papillæ of Monacanthus trossulus.
Fig. 18.—Dermal papillæ of Monacanthus hippocrepis (magn.)
These generally small bodies, as well as the large osseous scutes of the Rays, Sturgeons, etc., have been comprised under the common name Placoid scales; a term which deservedly is being abandoned.
Fig. 19.—Cycloid scale from the lateral line of Odax lineatus (magn.) Along the side of the body of osseous fishes runs a series of perforated scales, which is called the lateral line (Fig. 21). The perforating duct is simple at its base, and may be also simple at its outer opening (Fig. 19), or (and this is frequently the case) the portion on the free surface of the scale is ramified (Fig. 20). The lateral line runs from the head to the tail, sometimes reaching the caudal fin, sometimes stopping in front of it, sometimes advancing over its rays. It is nearer to the dorsal profile in some fishes than in others. Some species have several lateral lines, the upper one coasting the dorsal, the lower the abdominal outline, one running along the middle as usual. ​The scales of the lateral line are sometimes larger than the others, sometimes smaller, sometimes modified into scutes,
Fig. 20.—Cycloid scale from the lateral line of Labrichthys laticlavius (magn.)
sometimes there are no other scales beside them, the rest of the body being naked. The foramina of the lateral line are the outlets of a muciferous duct which is continued on to the head, running along the infraorbital bones, and sending off a branch into the præopercular margin and mandible. In many fishes, as in many Sciænoids, Gadoids, and in numerous deep-sea fishes, the ducts of this muciferous system are extraordinarily wide, and generally filled with mucus, which is congealed or contracted in specimens preserved in spirits, but swells again when the specimens are immersed in water. This system is abundantly provided with nerves, and, therefore, has been considered to be the seat of a sense peculiar to fishes, but there cannot be any doubt that its function is the excretion of mucus, although probably mucus is excreted also from the entire surface of the fish.
The scales, their structure, number and arrangement, are an important character for the determination of fishes; in most scaly fishes they are arranged in oblique transverse series; and as the number of scales in the lateral line generally corresponds to the number of transverse series, it is usual to count the scales in that line. To ascertain the number of longitudinal series of scales, the scales are counted in one of ​the transverse series, generally in that running from the commencement of the dorsal fin, or the middle of the back to the lateral line, and from the lateral line down to the vent or ventral fin, or middle of the abdomen.
Fig. 21.—Arrangement of scales in the Roach (Leuciscus ratilus): L I = Lateral line; t r = Transverse line. a. Transverse line from lateral line to ventral fin.
The scales of many fishes are modified for special purposes, especially to form weapons of defence or a protective armour, but the details of such modifications are better mentioned under the several families in which they occur. All scales are continually growing and wasting away on the surface, and it seems that some fish, at least,—for instance, Salmonoids—"shed" them periodically; during the progress of this shedding the outlines of the scales are singularly irregular.