Page:1902 Encyclopædia Britannica - Volume 25 - A-AUS.pdf/747

 A R T H R O P 0 D A

691

It becomes apparent from this enumeration that there are a good many important elements or ‘ ‘ meromes ” in an Arthropod metamere or somite which can become the subject of heteromerism or, to use a more apt word, of ‘ ‘ heterosis. ” It is all the more necessary to insist upon this inasmuch as there is a tendency in the discussion of the segmentation of the Arthropod body to rely exclusively upon the indications given by the tegumentary chitinous plates and the parapodia. 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 (raypa, a regiment). The word “ tagmosis ” is applicable to the formation of such regions. In the Chsetopods tagmosis always occurs to a small extent so as to form the head. In some Chaetopods, such as Chsetopterus and the sedentary forms, there is marked tagmosis, giving rise toT three or even more tagmata. In Arthropods, besides the head, w e 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 quasiindependent 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. A case which appears at first sight to be one of “autorhythmus” of the parapodia is that of the Diplopods (Julus, etc.), in which each apparent somite carries two pairs of legs or parapodia. It looks at first as though this were due to the independent multiplication of the legs ; but it is not. Contrary to what obtains in Apus, we find in Julus that there is a well-marked somite in the embryo corresponding to each pair of legs and that the adult condition arises from a fusion of the tegumentary meromes of adjacent somites (see below “Fusion”). 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 (Aranese) 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 1 The word “Law” is used in this summary merely as a con- the dislocation of single meromes and of whole series (tagmata) of venient heading for the statement of a more or less general pro- meromes is a common phenomenon. Thus, in fishes the pelvic fins may travel forwards to a thoracic and even jugal position in position.

consideration of tlie structure of the Arthropoda demands a knowledge of what may be called the laws of metamerism. These are not so fully ascertained or formulated as might be expected. The repetition of parts, which we note as metamerism, is, as Haeckel, Bateson, and others have recognized, only a special case of a tendency of the organic body to repetition of structural units or parts which finds one expression in bilateral symmetry. In certain worms (the Cestoidea and some Planarians) metameric segmentation is accompanied by the separation of the completed inetameres 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 Clnetopods, 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.y., 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 twentyone 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 1 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 homceomeric or heteromeric. The reference to 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” (jxipos). 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 dorsoventral 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 ccelomic 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).