42 



PLANORBIS.— LIMN^A PHYSA.—AURICULA. 



the warm parts of both continents : the second are subdivided into 

 the genera of Planorbis, Limn^a, Auricula, &c. 



22. The mollusks of the genus Planorbis have a thin shell 

 rolled upon the same plane, which is consequently discoid, and 

 the whorls very gradually increase in size {jig. 29). Their 



tentacles are long and 

 filiform, and the eyes 

 are placed at the internal 

 side of their base. These 

 gasteropods feed on vege- 

 table substances, and in- 

 habit stagnant waters ; m 

 winter they bury them- 



V / "^^ ^^^ selves in the mud, and 



^^ — ^ lie torpid. 



Fig. 29.— PLANORBIS. 23. The Limn^a {Jig, 



18, page 33) have the 

 same habits, and are almost always found in the same localities 

 as the planorbis, from which they are readily distinguished by 

 the form of their shell, the spire of which is oblong ; their tenta- 

 cles are thick and triangular. In the laying season, they are 

 often found in great numbers joined together, so as to form a sort 

 of chaplet. They swim on the back with the ventral disk extend- 

 ed on the surface of the water, and in winter they become torpid 

 like the planorbis. 



24. We give the name of Physa 

 to small mollusks found in foun- 

 tains, the shell of which {fig. 30) is 

 similar to that of the limnsea, but 



Fig. 30. 



PHYSA. 



very thin, without a thick lip, and 



without a fold on the columella. 



The animal, when it crawls or 

 swims, covers its shell with the two lobes of 

 its mantle. In other respects it closely re- 

 sembles the limnsea. 



25. The Auricula differ from the pre- 

 ceding by having la-rge oblique grooves upon 

 the columella of their shell, which is oval or 

 oblong {fig, 31). The name is derived 



Fig. 31. 



AURICULA. 



22. What are the characters of the genus Planorbis ? Upon what do ani- 

 mals of this genus feed ? Where are they found ? 



23. How is the genus Limnse'a distinguished from the genus Planorbis ] 

 What are the habits of this genus ? 



24. What are the characters of the genus Physa? 



25. How is the genus Auri'cula characterized ? (Auri'cula is formed 

 from the Latin, auris^ ear, and means a little ear.) 



