# 1911 Encyclopædia Britannica/Metamerism

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​METAMERISM (Gr. μετά, after, μέρος, a part), a technical term used in natural science: In chemistry it denotes the existence of different substances containing the same elements in the same proportions and having the same molecular weight; it is a form of isomerism.
In zoology, metamerism is the repetition of parts in an organized body, a phenomenon which is, as E. Haeckel, W. Bateson and others have recognized, only a special case of a tendency to repetition of structural units or parts which finds one expression in bilateral symmetry. It occurs in almost every group of the animal kingdom, but is most conspicuous in segmented worms, arthropods and vertebrates. In certain worms (the Cestoidea and some Planarians) metameric segmentation is accompanied by the separation of the completed metameres one by one from the older (anterior) extremity of the chain (strobilation), but it by no means follows that metameric segmentation has a necessary origin in such completion and separation of the “meres.” On the contrary, metamerism seems to arise from a property of organisms which is sometimes more (eumerogenesis) and sometimes less (dysmerogenesis) fully exhibited, and in some groups not exhibited at all. The most complete and, at the same time, simplest instances of metameric segmentation are to be seen in the larger Chaetopods, where some hundreds of segments succeed one another—each practically indistinguishable in structure from the segment in front or from that behind; muscles, right and left appendage or parapodium, colour-pattern of the skin, gut, blood-vessels, coelom, nephridia, nerve-ganglion and nerves are precisely alike in neighbouring segments. The segment which is least like the others is the first, for that carries the mouth and a lobe projecting beyond it—the prostomium. If (as sometimes happens) any of the hinder segments completes itself by developing a prostomium, the chain breaks at that point and the segment which has developed a prostomium becomes the first or head-bearing segment of a new individual. Compare such an instance of metameric segmentation with that presented by one of the higher Arthropods—e.g. the crayfish. Here the somites are not so clearly marked in the tegumentary structures; nevertheless, by examining the indications given by the paired parapodia, we find that there are twenty-one somites present—a limited definite number which is also the precise number found in all the higher Crustacea.
We can state as a First Law of metamerism or somite formation ​ that it is either indefinite in regard to number of metameres or somites produced, or is definite. Animals in the first case we call anomomeristic; those in the second case, nomomeristic. The nomomeristic condition is a higher development, a specialization, of the anomomeristic condition.
The Second Law, or generalization, as to metamerism which must be noted is that the meres or somites (excepting the first with its prostomium) may be all practically alike or may differ from one another greatly by modification of the various constituent parts of the mere or somite. Metamerized animals are either homoeomeric or heteromeric. The reference of the variation in the form of the essential parts contained in a “metamere” or “somite” introduces us to the necessity of a general term for these constituent or subordinate parts; they may be called “meromes" (μέρος). The meromes present in a metamere or somite differ in different annulate or segmented animals according to the general organization of the group to which the animal belongs. As a matter of convenience we distinguish in the Arthropod as meromes, first, the tegumentary chitinized plates called terga, placed on the dorsal aspect of the somites; second, the similar sternal plates. In Chaetopods we should take next to these the masses of circular and longitudinal muscular fibres of the body-wall and the dorso-ventral muscles. The latter form the third sort of merome present in the Arthropods. The fourth kind of merome is constituted by the parapodia or appendages; the fifth by the coelomic pouches and their ducts and external apertures (coelomo-ducts), whether renal or genital. The sixth by the blood-vessels of the somite; the seventh by the bit of alimentary tract which traverses it; and the eighth by the neuromere (nerve ganglion pair, commissures, connectives and nerve branches).
The Third Law of metamerism is that heteromerism may operate in such a way as to produce definite regions of like modification of the somites and their appendages, differing in their modification from that observed in regions before and behind them. It is convenient to have a special word for such regions of like meres, and we call each a tagma (τάγμα, a regiment). The word “tagmosis” is applicable to the formation of such regions. In the Chaetopods tagmosis always occurs to a small extent so as to form the head. In some Chaetopods, such as Chaetopterus and the sedentary forms, there is marked tagmosis, giving rise to three or even more tagmata. In Arthropods, besides the head, we find very frequently other tagmata developed. But it is to be noted that in the higher members of each great class or line of descent, the tagmosis becomes definite and characteristic just as do the total number of meres or somites, whilst in the lower grades of each great class we find what may be regarded as varying examples of tentative tagmosis. The terms nomotagmic and anomotagmic are applicable with the same kind of implication as the terms nomomeristic and anomomeristic.
The Fourth Law of metamerism (auto-heterosis of the meromes) is that the meromes of a somite or series of somites may be separately and dissimilarly affected by heteromerism. It is common enough for small changes only to occur in the inner visceral meromes whilst the appendages and terga or sterna are largely changed in form. But of equal importance is the independent “heterosis” of these visceral meromes without any corresponding heterosis of the body wall. As instances, we may cite the gizzards of various earthworms and the special localization of renal, genital and gastric meromes, with obliteration elsewhere, in a few somites in Arthropoda.
The Fifth Law, relating also to the independence of the meromes as compared with the whole somite, is the law of autorhythmus of the meromes. Metamerism does not always manifest itself in the formation of complete new segments; but one merome may be repeated so as to suggest several metameres, whilst the remaining meromes are, so to speak, out of harmony with it and exhibit no repetition. Thus in the hinder somites of the body of Apus the Crustacean we find a series of segments corresponding apparently each to a complete single somite; but when the appendages are examined we find that they have multiplied without relation to the other meromes of a somite: we find that the somites carry from two to seven pairs of appendages, increasing in number as we pass backwards from the genital segment. The appendages are autorhythmic meromes in this case. They take on a quasi-independent metamerism and are produced in numbers which have no relation to the numbers of the body-rings, muscles and neuromeres. This possibility of the independent metameric multiplication of a single merome must have great importance in the case of dislocated meromes, and no doubt has application to some of the metameric phenomena of Vertebrates.
The Sixth Law is the law of dislocation of meromes. This is a very important and striking phenomenon. A merome, such as a pair of appendages (Araneae) or a neuromere or a muscular mass (frequent), may (by either a gradual or sudden process, we cannot always say which) quit the metamere to which it belongs, and in which it originated, and pass by actual physical transference to another metamere. Frequently this new position is at a distance of several metameres from that to which the wandering merome belongs in origin. The movement is more usual from behind forwards than in the reverse direction; but this, probably, has no profound significance and depends simply on the fact that, as a rule, the head must be the chief region of development on account of its containing the sense organs and the mouth.
In the Vertebrata the independence of the meromes is more fully developed than in other metamerized animals. Not only do we get auto-heterosis of the meromes on a most extensive scale, but the dislocation of single meromes and of whole series (tagmata) of meromes is a common phenomenon. Thus, in fishes the pelvic fins may travel forwards to a thoracic and even jugal position in front of the pectoral fins; the branchiomeromes lose all relation to the position of the meromes of muscular, skeletal, coelomic and nervous nature, and the heart and its vessels may move backwards from their original metameres in higher Vertebrates carrying nerve-loops with them.
The Seventh Law of metamerism is one which has been pointed out to the writer by E. S. Goodrich. It may be called the law of “translation of heterosis.” Whilst actual physical transference of the substance of meromes undeniably takes place in such a case as the passage of the pelvic fins of some fishes to the front of the pectorals, and in the case of the backward movement of the opisthosomatic appendages of spiders, yet the more frequent mode in which an alteration in the position of a specialized organ in the series or scale of metameres takes place is not by migration of the actual material organ from somite to somite, but by translation of the quality or morphogenetic peculiarity from somite to somite, accompanied by correlative change in all the somites of the series. The phenomenon may be compared to the transposition of a piece of music to a higher or lower key. It is thus that the lateral fins of fishes move up and down the scale of vertebral somites; and thus that whole regions (tagmata), such as those indicated by the names cervical, thoracic, lumbar and sacral, are translated (accompanied by terminal increase or decrease in the total number of somites) so as to occupy differing numerical positions in closely allied forms (cf. the varying number of cervical somites in allied Reptiles and Birds).
What, in this rapid enumeration, we will venture to call the Eighth Law of metamerism is the law of homoeosis, as it is termed W. Bateson. Homoeosis is the making of a merome into the likeness of one belonging to another metamere, and is the opposite of the process of “heterosis”—already mentioned. We cite this law here because the result of its operation is to simulate the occurrence of dislocation of meromes and has to be carefully distinguished from that process. A merome can, and does in individual cases of abnormality, assume the form and character of the corresponding merome of a distant somite. Thus the antenna of an insect has been found to be replaced by a perfectly well-formed walking leg. After destruction of the eye-stalk of a shrimp a new growth appears, having the form of an antenna. Other cases are frequent in Crustacea, as individual abnormalities. They prove the existence in the mechanism of metamerized animals, of structural conditions which are capable of giving these results. What those structural conditions are is a matter for separate inquiry, which we cannot even touch here.
We now come to the questions of the production of new somites or the addition of new somites to the series, and the converse problem of the suppression of somites, whole or partial. We state as the Ninth Law of metamerism “that new somites or metameres are added to a chain consisting of two or more somites by growth and gradual elaboration— what is called ‘budding’—of the anterior border of the hindermost somite. This hindermost somite is therefore different from all the other somites and is called the ‘telson.’ However long or short or heteromerized the chain may be, new metameres or somites are only produced at the anterior border of the telson, except in the Vertebrata.” That is the general law. But amongst some groups of metamerized animals partial exceptions to it occur. It is probably absolutely true for the Arthropoda from lowest to highest. It is not so certain that it is true for the Chaetopoda, and would need modification in statement to meet the cases of fissiparous multiplication occurring among Syllids and Naidids. In the Vertebrata, where tagmosis and heterosis of meromes and dislocation of merones and tagmata are, so to speak, rampant, new formation of metameres (at any rate as represented by important meromes) takes place at more than one point in the chain. Such points are found where two highly diverse “tagmata” abut on one another. It is possible, though the evidence at present is entirely against the supposition, that at such points in Arthropoda new somites may be formed. Such new somites are said to be “intercalated.” The question of the intercalation of vertebrae in the Vertebrata has received some attention. It must be remembered that a vertebra even taken with its muscular, vascular and neural accessories is only a partial metamer—a merome—and that, so far as complete metameres are concerned, the Vertebrata do conform to the same law as the Arthropods. Intercalation of meromes, branchial, vertebral and dermal (fin-supports) seems to have taken place in Vertebrata in the fishes, while in higher groups intercalation of vertebrae in large series has been accepted as the only' possible explanation of the structural facts established by the comparison of allied groups. The elucidation of this matter forms a very important part of the work lying to the hand of the investigator of vertebrate anatomy, and it is possible that the application of Goodrich’s law (the seventh of our list) may throw new light on the matter.
In regard to the diminution in the number of somites in the ​course of the historical development of those various groups of metamerized animals, which have undoubtedly sprung from ancestors with more numerous somites than they themselves possess, it appears that we may formulate the following laws as the tenth, eleventh, twelfth and thirteenth laws of metamerism.
The Tenth Law is that individual somites tend to atrophy and finally disappear as distinct structures, most readily at the anterior and the posterior ends of the series constituting an animal body. This is very generally exhibited in the head of Arthropoda, where, however, the operation of the law is largely modified by fusion (see below). With regard to the posterior end of the body, the atrophy of segments does not, as a rule, affect the telson itself so much as the somites in front of it and its power of producing new somites. Sometimes, however, the telson is very minute and nonchitinized (Hexapoda).
The Eleventh Law may be stated thus: any somite in the series which is the anterior or posterior somite of a tagma may become atrophied, reduced in size or partially aborted by the suppression of some of its meromes; and finally, such a somite may disappear and leave no obvious trace in the adult structure of its presence. in ancestral forms. This is called the excalation of a somite. Frequently, however, such “excalated” somites are obvious in the embryo or leave some merome (e.g. neuromere, muscle or chitin-plate) which can be detected by minute observation (microscopic) as evidence of their former existence. The somite of the maxillipede (third post-oral appendage) of Apus cancriformis is a good example of a somite on its way to excalation. The third prae-oral and the praemaxillary somites of Hexapod insects are instances where the only traces of the vanished somite are furnished by the microscopic study of early embryos. The praegenital somite of the Arachnida is an example of a somite which is preserved in some members of the group and partially or entirely excalated in other cases, sometimes with fusion of its remnants to neighbouring somites.
The Twelfth Law of metamerism might very well be placed in logical order as the first. It is the law of lipomerism, and asserts that just as the metameric condition is produced by a change in the bodies of the descendants of unisegmental ancestors, so highly metamerized forms—i.e. strongly segmented forms with specialized regions of differentiated metameres—may gradually lose their metamerized structure and become apparently and practically unisegmental animals. The change here contemplated is not the atrophy of terminal segments one by one so as to reduce the size of the animal and leave it finally as a single somite. On the contrary, no loss of size or of high organization is necessary. But one by one, and gradually, the metameric grouping of the bodily structures disappears. The cuticle ceases to be thickened in rings—the muscles of the body-wall overrun their somite boundaries. Internal septa disappear. The nerve-ganglia concentrate or else become diffused equally along the cords; one pair of renal coelomoducts and one pair of genital coelomoducts grow to large size and remain—the rest disappear. The appendages atrophy or become limited to one or two pairs which are widely dislocated from their ancestral position. The animal ceases to present any indication of metameric repetition of parts in its entire structure. Degrees in this process are frequently to be recognized. We certainly can observe such a change in the posterior region of some Arthropods, such as the hermit-crabs and the spiders. Admitting that the Echiurids are descended from Chaetopoda, such a change has taken place in them, amounting to little short of complete lipomerism, though not absolutely complete.
Recent suggestions as to the origin of the Mollusca involve the supposition that such an effacement of once well-marked metamerism has occurred in them, leaving its traces only in a few structures such as the multiple gill-plumes and shell-shields of the Chitons and the duplicated renal sacs of Nautilus.
A further matter of importance in this connexion is that when the old metameres have been effaced a new secondary segmentation may arise, as in the jointed worm-like body of the degenerate Acarid, Demodex folliculorum.
Such secondary annulation of the soft body calls to mind the secondary annulation of the metameres of leeches and some earthworms. Space does not permit of more than an allusion to this subject; but it is worth while noting that the secondary annuli marking the somites of leeches and Lumbricidae in definite number and character are perhaps comparable to the redundant pairs of appendages on the hinder somites of Apus, and are in both cases examples of independent repetition of tegumentary meromes—a sort of ineffectual attempt to subdivide the somite which only prevails on the more-readily susceptible meromes of the integument.
The last law of metamerism which we shall attempt to formulate here, as the Thirteenth, relates to the fusion or blending of neighbouring somites. Fusion of adjacent somites has often been erroneously interpreted in the study of Arthropoda. There are, in fact, very varying degrees of fusion which need to be carefully distinguished. The following generalization may be formulated. “The homologous meromes of two or more adjacent somites tend to fuse with one another by a blending of their substance. Very generally, but not invariably, the fused meromes are found as distinct separated structures in the embryo of the animal, in which they unite at a later stage of growth.” The fusion of neighbouring meromes is often preceded by more or less extensive atrophy of the somites concerned, and by arrest of development in the individual ontogeny. Thus, a case of fusion of partially atrophied somites may simulate the appearance of incipient merogenesis or formation of new somites, and, vice versa, incipient merogenesis may be misinterpreted as a case of fusion of once separate and fully-formed somites.
A very complete fusion of somites is that seen on the head of Arthropoda. The head or prosoma of Arthropoda is a tagma consisting of one, two, or three prosthomeres or somites in front of the mouth and of one, two, three, up to five or six opisthomeres. The cephalic tagma or prosoma may thus be more or less sharply divided into two subtagmata, the prae-oral and the post-oral.  (E. R. L.)