& 



iving" 



Edible Mussel 



Introductory 27 



that live in fresh-water ponds and lakes, or the Edible 

 Mussel of our seashores. These are so much alike 

 in general structure, that 

 either will serve our present 

 purpose. Here the shell is 

 a bivalve — that is, it is 

 formed in two pieces {valves) 

 which are hinged together 

 by their upper edges. Having 

 seen that the shell of the 

 Snail was produced by the mantle, we shall not 

 be surprised to find that the Swan Mussel's mantle 

 is in two parts corresponding with the valves 

 of the shell. Within the two lobes of the mantle 

 there are four plates or leaves of very delicate tissue — 

 the gills — in two pairs. Between these again lie the 

 foot, the visceral mass, and the lips of the mouth. 

 This creature, though it has a mouth, has no head, 

 and it does not wander about in search of food, but 

 lies partly embedded in the mud with the shell gaping 

 slightly, and waits for its food to come to it. 



The gill-plates are covered with delicate filaments 

 (cilia) like the gossamer of small spiders, and, appar- 

 ently by the constant rhythmical movement of these, 

 currents of water are drawn in, passed over and between 

 the gills, where the blood circulating through their 

 tissues absorbs the oxygen, wdiilst the microscopic forms 

 of animal and plant life with which the waters abound 

 are filtered out and passed to the lips. This being 

 the method of their feeding, the bivalves lack the 

 horny jaws and the tooth-studded ribbon such as we 

 found in the Garden Snail. Occasionally examples 

 of the Swan Mussel are found combining the functions 



