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FIRST BOOK OF ZOOLOGY. 



GASTEBOPODS. 



Class Acephala (clams, oysters, mussels, etc.). Animals 

 whose bodies are protected by a mantle which secretes a bi- 

 valve shell, or a shell composed of two pieces. They have 

 no well-defined head, and hence the name Acephala, de- 

 rived from two Greek words, a, without, and cephale, head. 

 These animals are also called Lamellibranchiates, because 

 the gills form leaf-like membranes or plates on the sides of 

 the body ; the word being derived from a Latin and a Greek 

 word, lamella, a plate, and firanchia, gill. The following 

 figures represent a few animals of this class : 



