218 BERMUDA. 



The circulation is complete in the moUusca. Thei 

 heart is situated, in general, in the back, above the 

 intestinal canal. It is not contained in a true peri- 

 cardium, but in a muscular cell of the imperfect 

 diaphragm which separates the visceral cavity from 

 the branchia. 



The organs of respiration vary considerably, not 

 only with respect to their form and the place which 

 they occupy in the animal, but also with respect to 

 structure. 



The form of the body in the moUusca is extremely 

 various. It is frequently oval, more or less elon- 

 gated, convex above and flat beneath, as in the 

 genera Doris, Limcuc, &c. It is also sometimes 

 oval, and equally convex above and below, as in 

 the Sepice; elongated and cylindrical, as in certain 

 Loligines ; globular, as in the Octopodes. It is often 

 more or less compressed on the sides, as in the 

 Scylleee. In very many cases a large portion of the 

 body is rolled up in a spiral form. A considerable 

 number of these animals present a very distinct 

 separation between the head and the rest of the 

 body, as in the Octopodes; this distinction is some- 

 times much less marked, as in the genus Doris. The 

 distinction of neck, breast, abdomen, and tail is still 

 less obvirfus, the body forming only a simple mass. 



