MOLLUSKS 103 



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. 



99. 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. 



100. 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 



