8 MOLLUSCA. 



The form of the body in the Mollusca is extremely various. 

 It is frequently oval, more or less elongated, convex above, and 

 flat beneath, as in the genera Doris, Limax^ &c. It is also some- 

 times 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 Scyllcece. 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 sometimes much less marked, as in the genus Doris. The dis- 

 tinction of neck, breast, abdomen, and tail is still less obvious, 

 the body forming only a simple mass. It is seldom that the 

 body is furnished with organs of locomotion, properly so called, 

 although cutaneous expansions are sometimes remarked on the 

 sides, which are subservient to this purpose. 



The nervous system consists of a central part or brain, si- 

 tuated above the intestinal canal ; of ganglia for the different 

 organs of sense, as well as for the locomotive apparatus ; of a 

 few visceral ganglia, together with conducting filaments or nerves. 

 The brain consists of two similar parts, more or less connected, 

 and situated above the oesophagus. The ganglion of the organ 

 of sight communicates with the brain, as well as that of the or., 

 gan of hearing, when it exists. Besides the more or less im- 

 mediate connection which exists between the two parts of the 

 brain above the oesophagus, there is another which passes under 

 the oesophagus, thus forming a sort of ring. The visceral gan- 

 glia appear to be only two in number. The principal one is 

 commonly placed near the stomach, and sends forth filaments 

 to the intestinal canal, and others which communicate with the 

 brain, by means of the oasophageal ring. For the locomotive 

 apparatus and the organs of general sensation there is but a 

 single ganglion on each side, which communicates with the brain 

 by a cord. From this ganglion proceed the filaments that sup- 

 ply the musculo-cutaneous envelope, and especially those which 

 are subservient to general locomotion, such as the foot of the 

 Gasteropoda and Trachelipoda> the wings of the Pteropoda, &c. 



The circulation is complete in the Mollusca. The heart is 

 situate in general in the backj above the intestinal canal. It 

 is not contained in a true pericardium, but in a muscular cell of 



