CHAPTER VII. 



MOLLUSCA. 



Animals more or less resembling in structure the 

 Presh-water Mussel (type of the bivalve Molluscs) or 

 the Snail (type of the univalve Molluscs). 



Classes. — Lamellibranchiata or Oonchipeba (bivalves), 

 animals like the Eresh-water Mussel. 



(jASTEEOPODA (univalves), animals like Snails and 

 Winkles. 



Cephalopoda, or Cuttle-fishes. 



The Mollusca (soft-bodied animals), or shell-fisli, are 

 usually developed from a trochosphere, like that of 

 some worms, which afterwards- changes into a second 

 larval form, called a veliger larva, because it moves 

 by means of a ciliated fold, called the velum.-'; it 

 has a shell-gland to begia the secretion of the future 

 shell. The molluscs usually have a " shell " in some 

 form or other, secreted by the outer layer of the body, 

 and crawl by means of the " foot," a muscular ex- 

 pansion which is well seen in the snail. Their nervous 

 system consists of a paired cerebral ganglion, joined 

 by connectives with several other pairs of ganglia : 



' The velum is formed out of the anterior and ciliated 

 portion of the trochosphere. It is believed to be homologous 

 ■with the "wheel-bearing" ciliated disc of rotifers. In the 

 adult mollusc, it becomes converted into prominences sur- 

 rounding the mouth which are called the buccal lobes. 



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