. Elements of Comparative Anatomy. 358 COMPARATIVE ANATOMY. a curved dli^ection, and which is likewise covered by modified epithehum. Lacaze-Duthiers, Otocystes des Mollusques. Arch, de Zoologie. I. p. 97.— , J., Das Geliororgan etc. bei Pterotracliea. Zeitschr. f. wiss. XXV. Suppl.—V. Jheeing, Die Geliorwerkzeiige der Molluskeu. Erlangen, 1876.—SiMEOTH, Uobcr die Siuuesorgane unserer einlieim. Woiclitliiere. Zeitschr. f. w. Zool. Bd. XXVI. Alimentary Canal. § 275. Owing to the complete separation of the wall of the body from that of the enteric canal in the MoUuscaj the latter ca
. Elements of Comparative Anatomy. 358 COMPARATIVE ANATOMY. a curved dli^ection, and which is likewise covered by modified epithehum. Lacaze-Duthiers, Otocystes des Mollusques. Arch, de Zoologie. I. p. 97.— , J., Das Geliororgan etc. bei Pterotracliea. Zeitschr. f. wiss. XXV. Suppl.—V. Jheeing, Die Geliorwerkzeiige der Molluskeu. Erlangen, 1876.—SiMEOTH, Uobcr die Siuuesorgane unserer einlieim. Woiclitliiere. Zeitschr. f. w. Zool. Bd. XXVI. Alimentary Canal. § 275. Owing to the complete separation of the wall of the body from that of the enteric canal in the MoUuscaj the latter canal comes to be embedded in a coelom. It forms coils or loops in this cavity, as it is always longer than it; at the same time the characters of its anal opening are well worthy of note. We find, indeed, that it is only in the Placophora and Lamellibranchiata that it traverses the body in such a way that the aboral and anal ends of the body are the same. In the Scaphopoda, Gastropoda, Pteropoda, and Cephalo- poda it always ends at some distance from the aboral end of the body; it is looped or coiled. If we suppose that the gut was primitively arranged in a symmetrical manner, and that the anus was placed in the aboral region, and that, therefore, this change in the position of the anus was acquired afterwards, then we must also suppose that this arrangement obtained at a very far-distant period, inasmuch as there are no signs of it in the development of the indi- vidual. The real cause of this change in position be sought for in the universal possession of a shell. The development of the dorsal mantle and its shell, together with that asymmetrical mode of development of both of these structures which obtains in most MoUusca, makes it easy to see how they aifected the organism. We must distinguish two modes of develop- ment. In the first, the dorsal development of a part of the body which was protected by the shell, increased the space available for the growing gut,
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