CHAPTER XX 



THE MOLLUSCA 



The earthworm has been taken as an example of a bilaterally 

 symmetrical metamerically segmented coelomate animal. The 

 snail and the fresh-water mussel are examples of a group which 

 is undoubtedly related to the Annelida, but differs so much 

 from them in organisation that it requires a very extensive 

 study to understand the relationship. The fresh-water mussel 

 is bilaterally symmetrical, the adult snail is not, and neither are 

 metamerically segmented. In both the coelomic cavity is much 

 reduced, and both possess a number of structural features 

 peculiar to the phylum mollusca. It will be most convenient 

 to begin with the study of the fresh-water mussel, Anodonta 

 cygnea, for although this animal is deficient of some character- 

 istic molluscan organs, it serves better than the snail to illustrate 

 the main features of molluscan anatomy. As the anatomy of 

 Anodonta is fully described in Marshall and Hurst's " Practical 

 Zoology," it will not be necessary to give more than a general 

 account of its structure. 



Anodonta cygnea is a bivalve mollusc living partially im- 

 bedded in mud or sand at the bottom of fresh-water streams 

 and ponds. It is particularly abundant in some canals, such 

 as the Oxford and Birmingham Canal, into which it has made 

 its way from the Thames. Though by no means an active 

 animal, it is capable of ploughing its way slowly through the 

 mud by means of a powerful muscular organ called the foot. 

 When alive and undisturbed the two halves or valves of the 

 shell are generally kept slightly apart, and the muscular plough- 

 share-shaped foot may be seen protruding from the blunter 

 anterior end of the shell. When the animal is alarmed the foot 

 is withdrawn, and the two valves are closed and held tightly 

 together by powerful muscles, so tightly that unless the muscles 

 are severed the valves cannot be forced apart without injury. 

 When dead the muscles are relaxed, and the shell gapes. If a 



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