216 GENERAL ZOOLOGY 



In addition to dangers which threaten from without, Physa 

 is the subject of attacks by insidious enemies which live 

 within its body. Chief among these are the trematodes, 

 which pass their larval stages in snails and then pass out to 

 mature in other animals (page 179) . Against such parasites 

 Physa probably has no means of protection. 



Race Preservation. Snails are hermaphroditic and dur- 

 ing mating both the individuals taking part are usually 

 fertilized. Physa lays its eggs in a little drop of jelly (Fig. 

 86, A). This adheres to solid objects in the water, protects 

 the eggs, and furnishes nourishment for the young snails 

 developing in it. 



The development of fertilized snail eggs is very similar to 

 that of other metazoans .(Fig. 64), except in regard to the 

 cleavage. In very early cleavage stages cells of different 

 sizes are formed. Instead of lying one above the other, 

 they become spirally twisted ( = spiral cleavage). By the 

 direction of the twisting, the form of the shell which is to be 

 formed is indicated. Snails are of two sorts: right- and 

 left-handed, according to -whether the shell coils like a 

 right- or a left-handed screw. Physa is left-handed, but 

 most of the other common snails are not, and this fact may 

 usually serve for identification. It is of particular interest 

 that the direction of coiling of the shell is indicated in the 

 very earliest cleavage stages. 



Young Physas remain within the jelly for a time after 

 they are fully formed; then break out and are at once able 

 to shift for themselves. The parent takes no care of the 

 eggs after they have been laid. 



GENERAL REMARKS ON GASTROPODS 



Taking the animal kingdom as a whole, the most special- 

 ized and most successful animals are those which have be- 

 come able in their racial evolution to live on land. Among 

 the few groups that have become perfectly adjusted to 

 terrestrial life, snails have an interesting place because they 



