CHAPTER XVIII 



THE SWAN MUSSEL. MOLLUSCA 



Freshwater mussels may be found in most streams, 



canals, and large ponds in Britain, though they 



are often overlooked on account of their habit 



of burying themselves in the mud with at most a small part 



of the body projecting. The commonest of them is the 



Swan Mussel, Anodonta cygnea. When it is removed from 



the mud it is seen to be enclosed in a flat, dark-green 



shell, four to six inches long and roughly oval in outline, 



with one end (the front) rounded and the other more 



pointed. The shell consists of two similar pieces, known 



as valves, which lie one on each side of the animal 



joined by a hinge above the back, where their edges are 



almost straight. On being disturbed the mussel holds the 



valves tightly together, but when it is at rest in the water 



they gape somewhat, and at the hind end, which projects 



slightly from the mud, there may be seen between them 



two fleshy lobes enclosing an opening shaped like a figure 



of 8, through one of whose limbs a current sets into the 



shell, while through the other, the upper of the two, the 



water is driven out. At times the animal moves about, 



thrusting out a yellowish, ploughshare-shaped organ known 



as the foot, with which it ploughs its way through the mud 



at the rate of about a mile a year. Freshwater mussels are 



not unfit for food and are sometimes eaten. They are preyed 



upon by water-fowl and other animals, and in places are 



fished for on account of the pearls which they contain, 



which may be of considerable value. They are not killed 



by the freezing of the water even if they themselves be 



frozen solid, but can only survive a few hours of drought. 



The shell consists of an outer horny layer, \h&periostracum, 

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