PHYLUM MOLLUSCA 257 



Many snails feed mostly at night, and their eyes may be adapted 

 to dim light. 



Snails possess a sense of smell, since some of them are able to 

 locate food, which is hidden from sight, at a distance of eighteen 

 inches. We are not certain where the sense of smell is located, 

 but investigators are inclined to believe that the small tentacles 

 (Fig. 180) are the olfactory organs. A sense of taste is doubtful. 



There are two organs of equilibrium (statocysts) , one on either 

 side of the supra-cesophageal ganglia. They are minute vesicles 

 containing a fluid in which are suspended small calcareous bodies 

 (statoliths). Nerves connect them with the supra-oesophageal 

 ganglia. 



Locomotion. — The snail moves from place to place with 

 a gliding motion. The slime gland which opens just beneath 

 the mouth deposits a film of slime, and on this the animal moves 

 by means of wave-like contractions of the longitudinal muscular 

 fibers of the foot. Snails have been observed to travel two 

 inches per minute (Baker). 



Reproduction. — Some gastropods are dioecious; others are 

 monoecious. Helix is hermaphroditic, but the union of two 

 animals is necessary for the fertilization of the eggs, since the 

 spermatozoa of an individual do not unite with the eggs of the 

 same animal. The spermatozoa arise in the ovotestis (Fig. 181, 

 75); they pass through the coiled hermaphroditic duct (16) and 

 into the sperm duct; they then enter the vas deferens (79) and are 

 transferred to the vagina (25) of another animal by means of a 

 cylindrical penis (23) which is protruded from the genital pore. 



The eggs also arise in the ovotestis and are carried through the 

 hermaphroditic duct; they receive material from the albumen 

 gland (77) and then pass into the uterine canal ; they move from 

 here down the oviduct (18) into the vagina (25), where they are 

 fertilized by spermatozoa which were transferred to the seminal 

 receptacle (20) by another snail. In almost all other land pul- 

 monates impregnation is mutual, each animal acting during 

 copulation as both male and female. 



