120 SEA-SHORE LIFE 



however, we find tentacles, and also complex eyes each provided 

 with a lens and a well developed retina which, curiously enough, 

 hears a close general resemhlance to that of the vertebrates, for it is 

 turned inside out. Near the sides of the gills are organs that are 

 believed to serve in tasting the water, and there is also a pair of 

 primitive ear-like, or balancing, organs in the foot. Each of these 

 consists of a cavity lined with cells which bear long hairs, while the 

 interior of the cavity is filled with a gelatinous fluid, and contains 

 one or more stony concretions, or even grains of sand. It is proba- 

 ble that these primitive ear-like organs enable the mollusks to per- 

 ceive shocks and jars in the water, and it is certain that many of 

 them are capable of perceiving sound. 



The nervous system is quite simple and consists of three pairs 

 of ganglia, or masses of nerve cells, joined by nerve fibres. One 

 pair of ganglia is near the mouth and above the throat, another 

 pair is at the base of the foot, while the hindmost pair is under the 

 posterior end of the intestine. 



The early stages in the development of clams, oysters and 

 mussels, bear a general resemblance to those of certain groups of 

 worms, and it seems probable that mollusks and worms are de- 

 scended remotely from one and the same ancient stock, which 

 is now extinct. The egg develops into a little pear-shaped larva 

 closely resembling the tack-headed larva (Trocliophora) of worms 

 excepting that it is provided with a half-shell on either side. The 

 little moUusk swims through the water until the shell becomes too 

 heavy, and then it sinks to the bottom. A considerable number of 

 young lamellibranchs attach themselves to objects by means of a 

 byssus thread which is secreted by a gland in the foot. After 

 growing older, however, they cease to develop a byssus, and change 

 their habits. The young of the scallop, the long clam, and the pearl 

 oyster form these byssus threads, while the well grown animal does 

 not. The young of the fresh-water mussels are parasites, and bury 

 themselves for a time beneath the skin of fishes. 



A good general account of the structure of lamellibranchs is 

 given in Arnold Lang's "Textbook of Comparative Anatomy," Part 

 II, 1896, Macmillan Co.; and a systematic review by Woodward and 

 Tait in "Woodward's Manual of the Mollusca." 



All of the Lamellibranchs are aquatic, and the vast majority are 



