ARTHROPODA may arise, as in the jointed worm-like body of the degenerate Acarus, 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 Lunibricidfc 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 development of secondary metameric annulations within the area of a complete somite is not recorded among Arthropoda. _ It deserves distinct recognition as “hypo-metamerism” or formation of “somatidia.” The last law of metamerism which we shall attempt to formulate here, as the Thirteenth, relates to the fusion or blending of neighbouring somites. There are without doubt a large number of important generalizations, to be arrived at hereafter from the further study of the metamerism of Vertebrata and the peculiar phenomena exhibited by the dislocated meromes of the vertebrate’s somites. But this is not the place in which to attempt an outline of the special laws of vertebrate metamerism. 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. Yery 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 versd, incipient merogenesis may be misinterpreted as a case of fusion of once separate and fully-formed somites. Moreover, the two phenomena, merogenesis and fusion of meromes, actually occur side by side in some cases as in the pygidial shields of the Trilobitic and Limulus. The most commonly-noted cases of fusion of metameres are simply cases of the fusion of the tegumentary meromes—usually the terga only. Such a fusion has really very serious morphological importance, although superficial and readily acquired. It amounts to no more than the disposition of chitinous cuticle of equal thickness over the area of the terga of the somites concerned instead of the thinning of the cuticular deposit at the adjacent borders of the somites. The somites consequently lose their hinge ; they can no longer be flexed one on the other. Atrophy of the muscles related to such flexure necessarily follows. The mesosomatic portion of the posterior carapace of Limulus is no more than such a superficial fusion : the other meromes of the ankylosed somites (appendages, neuromeres, blood-vessels, &c.) are unaffected. Such, too, is the case with the pygidial shields of many Trilobites. On the other hand, the telson, which is joined in both these cases with the superficially fused segments by a fusion of its chitinous cuticle with that of its last-formed or budded somite, can only take part in the fusion as a result of arrest in its activity, which amounts to a late supervening atrophy. This arrest of the telson’s special bud-growth may take place very early, in which case we get a large telsonic shield and only a very few somites in front of it—none soldered to the telson as in Agnostus and Ilenus; or it may take place later when eight post-cephalic (opisthosomatic) somites have been formed as in Limulus—the last two incompletely. Or, again, thirty or more somites may have been produced before the arrest takes place and fifteen of these may be ankylosed with the telson to form the pygidial shield (Phacops, &c.). A more complete fusion of somites is that seen in 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 pne - oral and the post-oral. The shifting of the mouth backwards in Arthropoda so as to allow segments which once were post-oral to take up a prie-oral position, as prosthomeres, must be regarded as a case of dislocation of the meromes concerned (Sixth law), like the forward travelling of a fish’s pelvic fins. The anus does not appear to be liable to such dislocation in Arthropoda; but it certainly does travel away from its parental metamere in the Yertebrata, and may possibly do so in Cluetopoda when what must be called “lipomerism” or general obliteration of a metameric ordering of parts sets in. Such “lipomerism” must be supposed to have affected the Chsetopod
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ancestors of the Sipunculids, if those latter worms are to be traced genetically to the former, and the anus has shifted to the anterior third of the body. However that may be, the conception (first put forward by Lankester in 1875, 2) of the backward movement of the mouth in Arthropoda from the first somite to the second, third, or even fourth in the original post-oral series, is not only justified by embryological observation of the shifting in question, but finds its parallel in other instances of the law of dislocation of meromes. The fusion of the cephalic or prosomatic somites not only extends to tegumentary structures but to muscles, blood - vessels, and markedly to neuromeres. However, in the embryo of many Arthropoda the original neuromeres of the pne-oral somites can be distinguished, and in many cases the ccelomic cavities. Also it is a noteworthy fact that the tegumentary fusion (cephalic carapace, prosomatic carapace) appears sometimes to break down secondarily (e.g., squilla among Crustacea and Galeodes and Tarassidie among Arachnida). It appears that we must recognize as a principle that such fusions as the carapaces of Arthropoda can revert to the condition of free movable plates—and therefore we must not assume that forms with fused tergal plates are necessarily later, genetically, than allied forms with free movable tergal plates. When such reversion to a movable series of dorsal plates occurs it must not be assumed that any corresponding change takes place in the deeper meromes. On the whole, fusion and ankylosis of somites is not in itself necessarily a deep-seated or far-reaching process. It may or may not be accompanied by dislocation of important meromes or by lipomerism ; whilst, as for instance in the opisthosoma of the spiders, opiliones and acari, dislocation and lipomerism may occur without fusion of tegumentary plates, and with, on the contrary, a dwindling, and eventual atrophy of such plates. The general considerations as to metamerism set forth above will enable us to proceed to a consideration of the characters which distinguish the various groups of Arthropoda, and to justify the classification with which we started. The Theory of the Arthropod Head.—The arthropod head is a tagma or group of somites which differ in number and in their relative position in regard to the mouth, in different classes. In a simple Chaetopod (Fig. 1) the head consists of the first somite only; that somite is perforated by the mouth, and is provided with a prostomium or prse-oral lobe. The prostomium is essentially a part or outgrowth of the first somite, and cannot be regarded as itself a somite. It gives rise to a nerve-ganglion mass, the Fl ia ram nvirinp head °- L—D g ofregion the xu Hip tue ludxino and adjacent prostomial ganglion. Tn Chaetopods (the Polychseta) (Fig. 2), of an OTgochset cha-to we find the same essential structure, lum’; m, the mouth; a, prostomial ganglionbut the prostomium may give rise to the mass or archi-cerebrum; I, II, III, coelom of the two or more tactile tentacles, and to second, and third the vesicular eyes. The somites have first, somites. (From GoodQ. J. Micr. Sci. vol. well-marked parapodia, and the second rich, xl. p. 247.) and third, as well as the first, may give rise to tentacles which are directed forward, and thus contribute to form “the head.” But the mouth remains as an inpushing of the wall of the first somite. The Arthropoda are all distinguished from the Chietopoda by the fact that the head consists of one or more somites which lie in front of the mouth (now called prosthomeres), as well as of one or more somites behind it (opisthomeres). The first of the post-oral somites invariably has its parapodia modified so as to form a pair of hemignaths (mandibles). Twenty-five years ago the question arose as to whether the somites in front of the mouth are to be considered as derived from the prostomium of a Chsetopod-like ancestor. Milne-Edwards and Huxley had satisfied themselves with discussing and establishing, according to the data at their command, the number of somites in the Arthropod head, but had not considered the question of the nature of the prse-oral somites. Lankester (2) was the first to suggest that (as is actually the fact in the Nauplius larva of Crustacea) the prse-oral somites or prosthomeres and their appendages
