634 CRUSTACEA readily seen; but in the Decapoda, Stomapoda, and Isop:>da it cannot by any means be so easily demonstrated. " We know of no example of a ring in which we are able to distinguish all the pieces that we desire to enumerate. In some there is an absence of certain pieces from the places they should occupy; sometimes they are very intimately soldered together, so that we cannot detect even a trace of separation ; but in studying each of them separately, where it is most distinct, we shall be able to form a clear idea, and recognize its character in spite of its union with its neighbouring pieces. Moreover, although this analysis of the ring may not be always practicable, it is none the less true that it facilitates much the study of the exterior skeleton of articulated animals, and that it will permit us often to establish analogies where the greatest difference would at first sight appear to exist" (Milne - Edwards). Professor Huxley in his lectures 1 has somewhat simplified Milne -Edwards s viev of the crust acean segment. Taking for his type one of the body rings, or somites, of the common lobster (fig. 3), he points out that it con sists of a convex upper part, called the tergum (t), and a flatter in ferior side, the sternum (s), the angle of junction on each side of the tergum and sternum being produced downwards, forming what is termed the pleuron (or epimera) fyl.) For ordinary studies this view of a somite will be found sufficiently clear and simple. From the inner surface of these tegurnentary rings in the Crustacea various plates are given off which project into the internal cavity, constituting in some in stances cells and canals. According to Milne-Edwards, these processes are always developed at the points of union of two rings or of two neighbouring pieces of the same segment ; and this clis- position has obtained for them the name of apodema m0 reus). (fin. 4). They are the result , <*, , the epimera; ft, 6, 6, the sternum ;
- - / i i / , i c, f , (, the apodemata rising from the
OI a told Ot the integument- sternum and separating the insertions ary membrane which pene- oftlielc g s - trates more or less deeply between the organs, and which is strengthened with calcareous matter like the rest of the external structure, and always formed of two thin plates soldered together. They serve the office fulfilled by the bones in the Vertebrata, viz., that of affording solid surfaces to which the muscles aro attached. DIVISIONS OF THE BODY IN THE CRUSTACEA. By general consent and usage the body of a crustacean id divided into three regions, namely, head (or cephalon), thorax, and abdomen (see figs. 1 and 6), to each of which divisions seven out of the twenty-one segments are attributed. These terms are used as a matter of convenience, and are not assumed to be homologous with the grand divisions of head, thorax, and abdomen, in the Vertebrata. 2 All writers l (r rlunu S mar- 1 Medical Times and Gazette, 1857. 2 Mr C. Spence Bate, F.R.S., has, since 1855, earnestly advocated the adoption of the terms "pereion" and "pleon" as less objection able and more expressive than thorax and abdomen for the second and third divisions of the body (the term cephalon for the head being, of course, generally adopted). In his recent paper " On the Anatomy of the American King-Crab (Limulus polyphemus}," Trans. Linn. Soc. 1872, vol. xxviii. pp. 462-3, Prof. Owen has proposed the terms " cephaletron " and " thoracetron * for the anterior and posterior divisions of the body in Limulus, and "pleon" for the "tail-spine." agree in considering the last seven 3 segments to be abdominal. The only difference of opinion is as to the division of the first fourteen, how many are cephalic and how many thoracic. . Dr Dana is of opinion that Crustacea have no distinct head, but only a cephalothorax composed of the fourteen anterior segments, yet he admits that in the Edrioph thalmia the seven most anterior pairs of appendages are devoted to the mouth and organs of sensation, and in the extinct Pterygotus and in the Trilobita a distinct head certainly exists. Professor Huxley considers that the division in the Podophthalmia is indicated by the cervical fold and by the sudder change in the character of the appendages between the sixth and seventh somites, and an equally marked similarity between the latter and those of the eighth and ninth pair. So that according to this view we should have six cephalic, eight thoracic, and six abdominal somites (for Professor Huxley regards the telson as not being a true somite but a median appendage, like the post-oral plate in Pterygotus). But, as Dana well observes, the mouth organs may become legs, or the legs may become mouth organs by slight variations, therefore this line of division can hardly be maintained on the character of the appendages alone, seeing it does not hold good for other groups equally entitled to consideration, as is the Podophthalmia. 4 The subjoined table may serve to show these several views more clearly : Arrangement and Nomenclature of the Somites or Body- rings of tlie Crustacea, with their Appendages. 1st. Somite bearing the eyes. 2. bearing antennules, or 1st (inter nal) antennae. 3. ,, bearing antennte, or 2nd (exte;-- ,_ nal) antennre. 4. ,, bearing mandibles. 5. ,, bearing 1st maxillae. 6. bearing 2d maxillae. 7. bearing 1st maxillipeds. 8. bearing 2d 9. ,, bearing 3d ,, fbeartn earing organs of prehension, f ambulatory legs, or natatory r appendages. Each somite furnished with a pair of appendages, usually natatory, but variously modified, often branchi- ferous. ^ Caudal somite destitute of appendages, -^ "telson," or median appendage. C21 1 (JL.J THE APPENDAGES. Just as we find a typical number of twenty-one body segments to prevail among the Crustacea, so also in the appendages the type number of joints is seven, any departure from which is disguised by fusion of one or more joints together, the obsolete condition of others, or the depauperizatiou of the limb into numerous artiadi (?ee fig. 5). At the coxal joiut, or protopvdcte, each limb usually bifurcates ; firstly, there arises the external Jimb proper, or exopodlte, which is normally seven-jointed ; and secondly, the internal branch, or endopodite, which may serve as a mouth organ, a branchial appendix, a swimming organ, or a protection for the ova or the young. The appendages As other carcinologists are not agreed in regarding the long ensiform tail-spine of Limulus as representing the abdomen, we are still left in doubt, and have come to the conclusion that any satisfactory revision of the existing nomenclature must comprehend not one class only but the entire group of the Arthropoda, in the several clasees of which the terms proposed to be altered are now in general use. 3 That is reckoning the telson as the twenty-first segment. 4 In the Edriophthalmia the normal arrangement mostly is main tained, the fourth to the seventh pairs being epistomial appendages,
and the eighth to the fourteenth ambulatory organs.