CHAPTER IX 



THE MOLLUSCA 



WE now enter into a division of the animal kingdom which 

 is more popular than those we have so far dealt with ; 

 the " shell-bearing " section of it, at least, having been the 

 subject of the collector's attention for centuries. 



Although we have a goodly number of molluscs that are 

 terrestrial or lacustrine the vast majority are marine. 



The popular divisions are as follows : 



Bivalves. Those with a double shell. Example : 

 oyster, cockle, etc. 



Univalves. Single shells. Example : whelk, etc. 



Nudibranchi. Those with no outward shell. 



Cuttle-fishes. Example : octopus, squid, etc. 



But this is misleading. For instance, some of the so- 

 called " bivalves " are " multi valves " pholas, teredo, etc. 

 while " imi valve " might or might not cover those that 

 have an internal shell, such as the sea-hare, the squid, and 

 so forth. 



So, without going into full scientific arrangement, such as 

 " sub-class," " order," " sub-order," etc., we shall take 

 them in the large divisions, or " classes," or, as occasion 

 compels, " sub-classes." 



The Lamellibranchiata (Fig. 84). These are laterally 

 compressed molluscs, with the shell in two parts, joining 

 by a non-calcified portion forming a hinge or " ligament." 

 The animal within is symmetrical, and its back is against 

 the hinge of the shell. There is no distinct head ; when 



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