In Daphnia (Moina) rectirostris he saw, not without astonishment, that the spermatozoa resembled the star-formed seminal bodies of the higher Crustacea. In other species of Daphnia—as, for instance, in D. magna, D. sima, and D. longispina—they are small, rod-like, or conical bodies. It must, however, be remarked, that Daphnia rectirostris is considered by Dr. Baird as belonging to a different genus, and this great difference in the spermatozoa is an additional argument in favour of the separation.
The testis is simple and tubular; it generally lies along the intestine, in the same position as that occupied by the ovary, and opens, after a short vas deferens, immediately on the upper side of the terminal hooklets.
In D. rectirostris, indeed, it seemed to terminate in the rectum, which, therefore, acts as a sort of cloaca; but this is so unlike what takes place in other species, that it requires confirmation before it can be received as an undoubted fact.
Prof. Leydig considers that the recent investigations into the morphology of the arthropods have shown that their body consists of four parts—head, thorax, abdomen, and post-abdomen. This division, which was proposed by Erichson for the Crustacea, has not, however, been adopted by our greatest authorities on the subject. Zaddach, in his admirable monograph "Die Entwickelung des Phryganiden-Eies," divides the body into five parts: "Vorderkopf, Kopf, Brust, Leib, und Hinterleib," though it seems unnecessary to divide the head into two parts. With these exceptions, however, the opinion on this point is remarkably unanimous. Siebold and Stannius, (Anat. Comp., 1850); Milne Edwards, (Annales d. Sci. Nat., 1851, vol. xvi.); Dana, in his great work on Crustacea, 1852; Owen, (Lectures on Comparative Anatomy, 1855); and Huxley, (Memoir on Aphis, Linnean Trans., vol. xxi, 1858), all divide the body of the Crustacea into three parts—head, thorax, and abdomen (the two former having more or less completely coalesced into a cephalothorax). Most, however, if not all, of these eminent naturalists admit that the five posterior thoracic segments of Crustacea are homologous with certain segments which in Insects form part of the abdomen. To apply the word "thorax," however, in two groups so nearly allied as the Insects and the Crustacea, to two different parts of the body, is manifestly very confusing, and contrary to the first principles of nomenclature. The three thoracic segments of insects correspond, according to Erichson, whose views are generally adopted, to the three segments which in Crustacea bear the three pairs of maxillipeds; and the five segments, which carry the legs in decapods, belong in consequence to the abdomen. We ought, therefore, to alter our nomenclatures as regards the Crustacea, however inconvenient such a change may be; and we must for this group of arthropods, if at least we wish to keep the head and thorax distinct, add a fourth division—the post-abdomen—to the three generally admitted. It will no doubt be better, as a matter of convenience, to divide the insect body also, theoretically, into four parts, although practically there are but three, as in decapods.
