MOLLUSC A. 153 



The foot extends on each side beyond the body. From the 

 manner in which the blood is aerated, the auricle and ven- 

 tricle are placed longitudinally, the latter being anterior. 



GENUS ONCHIDIUM Cloak tuberculated. Snout en- 

 larged and emarginate. Tentacula two in number, with 

 eyes at the tips. This genus was instituted by Dr. Buchan- 

 an, (now Hamilton), in Lin. Trans., vol. v. p. 132, for the 

 reception of a species which he found in Bengal, on the 

 leaves of Typha Eiephantina. It is not, according to this 

 naturalist, " like many others of the worm kind, an herma- 

 phrodite animal ; for the male and female organs of genera-^ 

 tion are in distinct individuals. I have not yet perceived 

 any mark to distinguish the sexes, while they are not in 

 copulation ; as, in both, the anus and sexual organs are placed 

 in a perforation, (cloaca'communis), in the under part of the 

 tail, immediately behind the foot ; but, during coition, the 

 distinction of sexes is very evident, the penis protruding to 

 a great length, considering the size of the animal." 



2d Subdivision. 



Cloak and foot not parallel ; the viscera contained in a 

 spiral, dorsal protuberance, protected by a shell. 



This group includes the animals usually denominated 

 SNAILS. They bear a very close resemblance to the slugs. 

 The shield, however, has a thickened margin in front, des- 

 tined to secrete the matter of the shell. In the part corres- 

 ponding with the centre of the shield in the slugs, there i, 

 {as Cuvier has characteristically termed it), a natural rup- 

 ture, through which the viscera are protruded into a conical 

 bag twisted spirally. In this bag are contained the principal 

 viscera, the liver occupying its extremity. The body of the 

 animal is attached to the pillar of the shell by a complicated 

 muscle, which shifts its place with the growth of the animal. 



