GASTROPODA PULMONATA. 51 



considerable depths in Lake Constance). The anus and renal open- 

 ing are placed near the respiratory aperture, sometimes in the 

 respiratory cavity itself. The generative organs open some way in 

 front, but on the same side. In the forms with a left-handed spiral, 

 the respiratory orifice, anus and generative opening are on the left 

 side. Some Pulmonates are naked, or possess only rudiments of the 

 shell in the dorsal integument ; others carry a relatively thin and 

 usually right-handed shell. Physa, Planorbis, and Clausilia alono 

 present a left-handed spiral. A true operculum is absent. On tho 

 other hand, many forms secrete temporarily a winter operculum. 



While the Pulmonates (with some exceptions) resemble the Proso- 

 branchs in the position of the heart behind the respiratory organs, 

 in the arrangement of other organs, e.g., the nervous system, they 

 more resemble the Opisthobranchs. The dentition consists of 

 an unpaired, horny, and usually longitudinally-ribbed upper jaw 

 (which, however, may be absent) and of a radula, which is covered 

 with a great number of toothed plates in longitudinal and trans- 

 verse rows. All are hermaphrodite. A few, e.g., species of Clausilia 

 and Pupa, are viviparous. Most Pulmonates, however, lay eggs, 

 either as in the fresh-water forms united in tubular or flat masses on 

 water-plants, or as in the terrestrial forms in damp places, each ono 

 being surrounded by a protecting calcareous shell. The ovum is 

 always contWied in a large mass of albumen,' which serves as 

 nourishment to the developing embryo. 



I. Basommatophora. 



The eyes lie at the base of the two tentacles. Present many 

 resemblances to the Tectibranchiata. 



Fam. Linmaeidae. Limnceus auricularis Drap., (Pond snail); L. stagnalis 

 0. Fr. Mttller ; Physa fontinalis L., Planorbis corneus L., Ancylus Jturiatilis 

 Blainv. 



Fam. Auriculidae. Auricula Judce Lam., _4. Midcs Lam., Caryckium mini- 

 mum, 0. Fr. Mull. 



II. Stylommatophora. 



The eyes lie at the tips of two usually retractile tentacles (posterior 

 tentacles). 



Fam. Peroniadse (Amphipneusta). Lung behind heart (Opisthopulmonate). 

 Pcronia verrueulata Cuv., Veronicella Blainv., Onchidium Buchan. 



Fam. Limacidae (Naked snails). Arion Fer. Sexual opening beneath tho 

 respiratory orifice in front of the middle of the dorsal shield. Back without a 



