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, 
bears a close general resemblance 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 (T'rochophora) of worms 
excepting that it is provided with a half-shell on either side. The 
little mollusk 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 
