MOLLUSC A OR UNIVALVES. 141 



Family 3. — Phyllidida* 



Some of the shells comprised in this family 

 differ from all other univalves, in being com- 

 posed of moveable pieces at the back, formed 

 to facilitate the motion of the animal. They 

 are shaped like a boat, and adhere to rocks co- 

 vered by the sea, the backs of turtle, and other 

 marine objects ; and so fast do they retain their 

 situation, by exhausting the air beneath their 

 bodies, that it requires a strong lever to detach 

 them, frequently breaking the shell before the 

 animal can be forced from its position. The 

 others are particularly simple in form, present- 

 ing, in fact, scarcely any thing more than a 

 conical cup or deep dish. They adhere to rocks 

 and buildings washed by the sea ; and although 

 of a dull colour outside, are often very beau- 

 tiful in the interior. — This family consists of 

 five genera. 



14. Phyllidea. — Shell none. Animal, 

 body long, four tentacula attached to the head. 

 — 4 species. 



* Les Phyllidiem, Lamarck. 



