470 The Animal Kingdom 



THE CLASS GASTROPODA 



Typically the gastropods have the soft body enclosed within a 

 spirally coiled shell. The most familiar of the gastropods are the 

 snails which live in both salt and fresh water and in most habitable 

 spots on land (Figs. \S6, a,c,d,j,h>j,k>m,p). 



Due to the coiling of the shell, the viscera of the snails is asym- 

 metrical, notwithstanding the fact that the protruding foot is bilaterally 

 symmetrical. Anteriorly the head region is well supplied with sense 

 organs, including tentacles. Eyes are usually present, but vary in their 

 exact position. 



The mouth is provided with the rasping radula; the long looped 

 digestive tract opens on the foot through the anus not far from the 

 mouth. A heart is present and enclosed in the pericardial sac ; the 

 nervous system has concentrations of nervous tissue in the head re- 

 gion. There is usually only one kidney. In general, marine forms 

 and some fresh-water species respire by means of gills; most ter- 

 restrial forms as well as some of the fresh-water and marine forms 

 have the mantle cavity modified to serve as a "lung" for respiration. 

 These latter are known as puhnonate snails. 



Some snails are monoecious, others are dioecious. In the mon- 

 oecious as well as dioecious forms, cross-fertilization is the rule. In 

 most fertilization is internal, but in others the sperms and eggs are 

 simply discharged into the water. Many of the snails lay astonishing 

 numbers of eggs. At times, there are complex mating behavior pat- 

 terns which precede the actual copulation. In most terrestrial and fresh- 

 water forms, development is direct, but in many marine forms, develop- 

 ment is through a veliger larval stage. 



Most of the snails glide or crawl along the substrate on the broad 

 foot. Often the anterior part of the foot is placed forward, and the 

 remaining portion pulled after it in a looping motion. Some others have 

 a groove down the foot and advance by having first one side than the 

 other take hold. The firm hold of the snail to the substrate is due both 

 to tiny depressions along the foot and to slime secreted by special 

 glands. Some marine forms, the pteropods, have the edges of the foot 

 prolonged into winglike structures ; these forms are able to float on the 

 surface of the water. 



Except for a few carnivorous forms, most snails are herbivorous. 

 They rasp ofT their food by means of the radula. This is a flattened 

 ribbonlike structure bearing rows of teeth. It is attached at either 



