CHAPTER XIX. 

 PHYLUM X: MOLLUSCA. 



235. General Survey. The animals in this group are 

 more complex in their organization than any we have 

 yet studied. They include snails, mussels, clams, devil- 

 fish, and squid. They differ radically from the echino- 

 derms in that they are not radially symmetrical; they 

 differ from the Annulata in that they do not have seg- 

 mented bodies. They are not often attached, though 

 most of them are sluggish and very poor movers. They 

 are largely aquatic or frequenters of moist places, though 

 some are able to withstand drouth for a time by reason 

 of their shells. Wherever exposed, the skin of mollusks 

 is soft and slimy, and within the shell the whole body is 

 soft. From this fact their name is derived. 



In general shape and external appearance the mollusks 

 are very varied. There is little in the outer features of 

 the clam and the snail and the devil-fish to indicate that 

 they are related, but there are some fundamental things 

 in which they do agree, and these differences in body are 

 but the evidence of the very different mode of life to 

 which they have become adjusted. In addition, the 

 interesting points in which the mollusks agree are : 



i . There is a fold of the body wall and skin, called the 

 mantle, which develops about the body in such a way 

 as to enclose a space (mantle chamber) in which the breath- 

 ing structures (gills) lie. The outer surface of the mantle 

 usually has glands that secrete a limy material which forms 



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