86 ANIMAL FORMS 
may be those of taste; but experiments along the line are 
difficult to perform, and our knowledge of this subject is 
far from complete. The same is true of hearing. Certain 
organs, interpreted as ears and located in the foot, have 
the form of two hollow sacs, containing one or more solid 
particles of sand or lime, whose jarrings, when effected by 
sonorous bodies, may result in hearing. On the other hand, 
it is held by some that they, like the semicircular canals of 
higher animals, may regulate the muscular movements 
which enable the animal to keep its balance. 
88. Egg-laying habits and development.— The egg-laying 
habits of the gasteropods differ almost as widely as their 
haunts. The terrestrial forms lay comparatively few eggs, 
ranging in size from small shot to a pigeon’s egg in some 
of the tropical species. These are buried in hollows in the 
ground or under sticks and stones, and after a few weeks 
hatch out young snails having the form of the adult. The 
same is also true of most of the fresh-water snails, which 
lay relatively smaller eggs embedded in a gelatinous mass 
frequently found attached to sticks and leaves, or on the 
walls of aquaria in which they are confined. Many marine 
species construct capsules of the most varied patterns 
which they attach to different objects, and in these the 
young are protected until they hatch. In the limpets and 
many of the chitons the eggs are laid by thousands directly 
in the water, and after a short time develop into free-swim- 
ming young, differing considerably from the parent in ap- 
pearance. Those escaping the ravages of numerous enemies 
finally settle down in a favorable situation and gradually 
assume the form of the adult. 
89. Age, enemies, and means of defense of lamellibranchs 
and gasteropods.— How much time is consumed by the young 
in growing up, and the length of time they live, are ques- 
tions generally unsettled. It is said that the oyster requires 
five years to attain maturity, and lives ten years; the fresh- 
water clam develops in five years, and some species live from 
