486 PHILOSOPHY OF ZOOLOGY. 



1st Subdivision. 



Heart entire, and detached from the rectum. 



This group, forming the Pectimhranchia of Cuvier, in- 

 fUides nearly all the marine gasteropoda which have spiral 

 univalve shells. It likewise contains a few species which 

 inhabit the fresh water. 



The foot is usually fortified above, on its posterior ex- 

 tremity, with a corneous plate, which acts as a lid to the 

 shell, when the animal is withdrawn into the cavity. 

 The anterior extremity is in some of the species double. 

 The anterior margin of the cloak forms a thick band, or 

 arch, rising from the foot, behind which is the portion of 

 the body that is always contained in the shell, and which 

 is covered with a very thin skin. Between the margin of 

 the cloak and foot is situate the head, supported on a short 

 neck. The tentacula are two in number, bearing eyes at 

 their base, or on short lateral processes, which have some 

 claims to be considered as tentacula. The hood is frequent- 

 ly emarginate, and sometimes fringed. The mouth is 

 more or less in the form of a proboscis, in some cases armed 

 within with spinous lips, or furnished with a long narrow 

 spiral tongue, armed with spines, as in the common peri- 

 winkle. The nature of this kind of tongue, the spiral ex- 

 tremity of which is free and lodged in the abdomen, is not 

 well understood. 



The entry to the gills is by a large aperture between the 

 margin of the cloak and neck, at the middle, or towards the 

 right side. These are contained in a cavity on the back of 

 the animal, and consist of leaves arranged in one or more 

 rows, which adhere to the walls of the cavity. At the en- 

 trance of this cavity is the anus and oviduct. 



The male and female organs are considered not only as 

 distinct, but as occurring on different individuals. The 

 evidenpe in support of this opinion is hi nuiny cases com- 



