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. — 

 E.ANKE, J., Das Geliororgan etc. bei Pterotracliea. Zeitschr. f. wiss. Zool.- 

 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 m.ust 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, which formed more 

 or less complicated loops or coils. At first, 

 of course, this only enabled it to depart 

 from its straight course. In the second, the 

 development of a mantle-cavity, the appear- 

 ance of which was correlated with that of the mantle and shell, 

 affected the position of the gut. When the mantle-cavity is 

 developed in the hinder region of the body, as it is in the Thecoso- 

 inatous Pteropoda and the Cephalopoda (Fig. 190), the position of 



Fig. 190. 

 relations 



Diagram of the 



of the enteric 



canal. A In Pteropoda. 



B In Cephalopoda. 



'p Foot, t Arms or Tentacles. 



Ir Branchiee. 



