178 MOLLUSCA. 



univalve shells. It likewise contains a few species which 

 inhabit the fresh water. 



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

 mity, with a corneous plate, which acts as a lid to the shell, 

 when the animal is withdrawn into the cavity. The ante- 

 rior extremity is in some of the species double. The ante- 

 rior 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 tenta- 

 cula are two in number, bearing eyes at their base, or on 

 short lateral processes, which have some claims to be con- 

 sidered as tentacula* The hood i s frequently emarginate, 

 and sometimes fringed. The mouth is more or less in the 

 form of a proboscis, in some cases armed within with spin- 

 ous lips, or furnished with a long narrow spiral tongue, armed 

 with spines, as in the common periwinkle. The nature of 

 this kind of tongue, the spiral extremity 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 oc- 

 curring on different individuals. The evidence in support 

 of this opinion is in many cases complete. The penis is in 

 some external, and incapable of being withdrawn, while in 

 others it is retractile, and situate in a cavity in the right ten- 

 taculum. The body of the animal is attached to the shell 



