644 EVOLUTION OF ANIMALS Part V 



In most species, there is a hard plate on the upper surface of the foot that is 

 last to be drawn into the shell. This is the operculum that acts as a stopper to 

 evaporation and keeps out intruders. The small opercula of snails were the 

 original eyestones passed between the eyelid and eye to bring out foreign 

 bodies. 



Feeding. Snails scrape surfaces with the rasping tongue or radula; when a 

 garden snail is rasping cabbage the sound can be heard several feet away 

 (Fig. 31.11). The radula is a horny ribbon with ridges and teeth on its upper 

 surface and beneath it is a cartilage which can be pushed forward against what- 

 ever the snail is feeding upon. The radula is then pulled back and forth over 

 the cartilage to rasp a green leaf, the skin of a tadpole, seaweed, or films of 

 algae and bacteria depending upon the snail's habits. A great many snails are 

 carnivorous and in these the radula is at the end of a proboscis which can be 

 extended through a hole bored in a shell. The familiar and unpopular "drills" 

 are snails that rasp holes in the shells of edible clams and oysters and other 

 bivalves whose pierced shells are common on many beaches (Fig. 31.12). Sea 

 slugs, beautiful though too soft, feed upon sea anemones likewise soft (Fig. 

 31.13). 



Relationships. Snails are the hosts of immature worms, including the highly 

 injurious flukes that as adults are parasites in birds and mammals. Both fresh- 

 and salt-water snails are eaten in great numbers by fishes and shore birds. Any 

 one who examines the stomachs of common fresh-water fishes known as suckers 

 will find plenty of small snails swallowed whole, with shells being slowly 

 dissolved by the powerful digestive juices. In the stomach of a mullet (a 

 name given to many small bottom-feeding fishes in fresh and salt waters), one 

 investigator found 35,000 little marine snails. Snails are generally unimportant 

 among human foods, but at European shore resorts roasted periwinkles are 

 sold in bags like peanuts; and steaks from the foot of abalones are sold in 

 California markets. 



Reproduction. In about half the species of snails there is a fully function- 

 ing male and female reproductive system in each individual, but even so, these 

 snails mate and cross fertilization occurs as it does in a similar situation in 

 earthworms. Fresh-water snails produce relatively few eggs in blobs of crystal 

 clear jelly deposited on submerged stones and on the undersides of floating 

 leaves. Marine snails produce great numbers of eggs. The sea hares (or sea 

 slugs, Tethys calijornicus) of the California coast lay their eggs in gelatinous 

 strings. By counting and computing them, the MacGinities of the Kerckhoff 

 Marine Laboratory, California, found that one of these sea hares produced 

 478 millions of eggs in four months and one week. The animal, obviously kept 

 in captivity, weighed five pounds and 12 ounces. 



In many mollusks, sex variations occur in the same individual. Young 

 marine snails of the genus Crepidula, commonly called boat shells, function at 



