460 PHILOSOPHY OF ZOOLOGY. 



The Bucdmim ackula of Mulleu, a native of England^ 

 belongs to this genus ; and Hkewise the Hdix octona of 

 LiNNsus, erroneously considered as a native of Britain. 



Order II. — Aquatic. 



The aquatic pulmoniferous gasteropoda have their resi- 

 dence constantly in the water. They possess two tentacula 

 only. These are usually flattened, incapable of being with- 

 dra^vn, and having the eyes at the internal base. The food 

 consists of aquatic plants. Respiration can only take place 

 at the surface of the water, to which the animals occasional- 

 ly ascend, to expel from the pulmonary cavity the vitiated 

 air, and replenish it with a fresh supply. The sexes are 

 united. The spawn, which is in the form of a rounded ge- 

 latinous mass, containing many ova, is deposited on aquatic 

 plants under water. Previous to hatching, the foetus must 

 be aerated by means of some branchial arrangement. 



1st Subdivision. 



Body protected externally by a shell. 



The animals belonging to this subdivision bear a very 

 close resemblance to the snails, in the structure of their bo- 

 dy, and the form of their shell. 



1st Tribe. 



The protecting shell spirally twisted. 



A. Shell turreted. 



]. WJiorls dextral. 



24. Lymneus. — Aperture of the shell having the right 

 lip joined to the left at the base, and folding back on the 

 pillar. 



The tentacula are lanceolate and depressed. The mouth 

 is furnished with three jaws ; the lateral ones simple ; the 

 upper one crescent-shaped, and emarginate. The male and 

 female organs, though intimately connected internally, 

 have their external orifices separated to a considerable di&- 



