463 PHILOSOPHY OF ZOOLOGY. 



manteau, au dessous des tentacules, et s''agite sans cesse 

 d'un mouvemeiit vermiculaire." But little doubt, we tliink, 

 can be entertained that this naturalist had been dev^^ived by 

 some of the parasitical leeches which infest the aquatic pul- 

 monifera, and that, instead of breathing by means of tubu- 

 lar gills, the animal of the L. auricularius possesses, like 

 those which it resembles in other characters, a pulmonary 

 cavity. 



2. Whorls sinistral. 



25. Physa. — Pillar-lip destitute of a fold. 



The external appearance of the animal is similar to the 

 Lymneus ; but the margin of the cloak is loose, divided in- 

 to lobes, and capable of being reflected over the surface of 

 the shell near the mouth. This genus was instituted by 

 Draparnaud. The Bulla Jhntinalis of British authors is 

 regarded as the type of the genus. 



26. Aplexa. — Pillar-lip, with a fold. 



This genus was instituted by us for the reception of the 

 Bulla hypnorum and rivalis of British writers. The shell 

 is more produced than in the physa. The cloak of the ani- 

 mal is incapable of being reflected on the shell, and its mar- 

 gin is destitute of lobes. 



B. Shells depressed. 



The spires revolve in nearly the same horizontal line. 

 The tentacula are long and filiform. 



27. Plakorbis. — Cavity of the shell entire. 



This is another sinistral genus ; the vent, pulmonary ca- 

 vity, and sexual organs, being on the left, and the heart on 

 the right side. The P. corneus, the type of the genus, 

 pours forth, when irritated, a purple fluid from the sides, 

 between the foot and the margin of the cloak. 



28. Segmentina. — Cavity of the shell divided. 

 Externally, the shell appears similar to planorbis ; but in- 

 ternally, it is divided by testaceous, transverse partitions, in- 



