MOLLUSKS 263 



pericardium or sack. The intestine passes directly through the 

 heart, a condition found in no other group of animals. 



Locomotion.^ — Locomotion may be observed if mussels are 

 kept in an aquarium. The fleshy foot is thrust down into the 

 mud or sand and then contracted. This action pulls the clam 

 forward for a short distance. Locomotion is thus very slow. 



The Nervous System. — Although the mussel appears to have no organs 

 of sight or hearing, yet it is provided with a complicated nervous system 

 which appears to have much to do with muscular activity. Three large 

 collections of nerve cells (called ganglia) are found, one near each adductor 

 muscle and one near the foot (see diagram). 



Early Development. — The early life history of most mollusks includes 

 a free-swimming stage before the young possess a shell. At this time the 

 tiny larva swims by means of cilia, near the surface of the water. The 

 fresh-water mussel at an early stage attaches itself to the gills of a fish, 

 thus living for a time as a paraisite. Eventually all bivalve mollusks come 

 to live near the bottom, where they are near a source of food supply. 



The Oyster. — The chief difference between the oyster and the 

 clam lies in the fact that the oyster is fastened by one valve to 

 some solid object, while the clam and the fresh water mussel move 

 about. This results in an asymmetry in the shell of the oyster. 



Oysters are never found in muddy localities, for in such places 

 they would be quickly smothered 

 by the sediment in the water. They 

 are found in nature clinging to 

 stones or on shells or other objects 

 which project a little from the bottom. 

 Here food is abundant and oxygen is 



, Shell of oyster, snowing asymmetry. 



obtained from the water surroundmg 



them. Hence oyster raisers throw oyster shells into the water to 



hold the young off the muddy bottom. 



In some parts of Europe and this country where oysters are 

 raised artificially, stakes or brush are sunk in shallow water so 

 that the young oyster, which is at first free-swimming, may escape 

 the danger of smothering on the muddy bottom. 



After the oysters are a year or two old they are taken up antl 

 put down in deeper water as seed oysters. At the age of three 



1 See Hunter and Valentine, Manual, page 142. 



