| OCR Text |
Show 6 CLASS I. CRUSTACEA. The Crustacea are articulated animals, with articulated feet, respiring by means of branchire, protected in some by the borders of a shell, and external in others, but which are not inclosed in special cavities of the body, and which receive air from openings in the surface of the skin. Their circulation is double, and analogous to that of the Mollusca. The blood is transmitted from the heart, which is placed on the hack, to the different parts of the body, whence it is sent to the branchire, and thence back again to the heart( I). These branchire, sometimes situated at the base of the feet, or even on them, and at others on the inferior appendages of the abdomen, either form pyramids composed of laminro in piles, or bristled with setrn or tufted filaments of simple ones, and even appear in some cases to consist wholly of hairs. Some of the Zootomists, Baron Cuvier in particular, had already made known to us the nervous system of various Crustacea of different orders. The same subject has lately been thoroughly examined by Messrs Victor Audouin and Milne Edwards in their third Memoir on the Anatomy and Physiology of these animals-Ann. des Sc. Nat. XIV, 77 ,-and all that is now wanting to complete their researches, is the publication of those made by M. Straus on the Branchiopoda and the Limuli in particular, which they have not noticed. ( 1 ) See the order Decapoda. CRUSTACEA. 7 "The nervous system of the Crustacea submitted to our observation, say they, presents itself in two very different aspects, which constitute the two extremes of the modifications visible in that class. Sometimes, as in th~ Talitrus, this apparatus is constituted by numerous similar nervous inflations, · arranged in pairs, and united by cords of communication in such a way as to form two ganglionic chains, separated from each other and extending throughout the length of the animal. ·At others, on the contrary, it is solely composed of two gan· glions or knotty enlargements, dissimilar in form, volume, and arrangement, but always simple and azygous, and situated, one in the head and the other in the •thorax. Such is the case in the Ma ia. "These two modes of organization, at the first glance, certainly seem essentially different, and if the study of the ner· vous system of the Crustacea were limited to these two animals, it would be extremely difficult to recognize the analogy between the central nervous mass in the thorax of the Maia, and the two ganglionic chains which occupy the same region of the body in the Talitrus. But if we remember the various facts detailed in this memoir, we necessarily arrive at this remarkable result." They were led to it by the exact and careful study of the nervous sys~em of various intermediate Crustacea, forming so many links of the series, such as t.he Cymothore(l ), the Phyllosomre( 2), Astacus(3), Palremon and Palinurus. They have also supported their positions by the observations of Cuvier and those of M. Treviranus. The consequence deduced by th~m is, that notwithstanding this difference, the nervous system of the Crustacea is formed of the same elements, which, insulated in some and uniformly distributed throughout the length of the body, present in others, various degrees of centralization, at first from without inwardly, and then in a longitudinal di- (1) lsopoda. (2) Stomapoda. (3) }'or this subgenus and the two following subgenera see the Decapod~~o Macroura. |
