270 SNAILS. PHYLUM MOLLUSCA 



of each is protruded and inserted into the vagina of the other. 

 The spermatophores are ejected and stored in the spermatheca, 

 where their covering is dissolved and the sperms set free. At 

 some time about June the ovotestis ceases forming sperms and 

 makes ova instead. These are fertiUsed at the base of the 

 hermaphrodite duct in a small chamber, the receptaculum 

 seminis, by sperms which have swum up the common duct from 

 the spermatheca. The eggs are then surrounded by albumen 

 secreted by the albumen gland, and after passing down the female 

 portion of the common duct, are laid all together in holes in the 

 ground. This happens in July and August. Development is 

 direct, and small snails hatch after about twenty-five days. 



SWAN MUSSELS 



Freshwater mussels may be found in streams, canals, and large 

 ponds in most parts of 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 are the swan mussels, Anodonta cygnea and A. anatina, 

 which differ only in small details of the shape of the shell. When 

 one is removed from the mud it is seen to be enclosed in a fiat, 

 dark-green shell, four to six inches long and roughly oval in out- 

 line, 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 flows 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 some- 

 times eaten. They are preyed upon by water-fowl and other 

 animals, and in places are fished for on account of the pearls 



