IV. GASTEROPODA: PULMONATA. 



383 



Order III. Pulmonata 



In several respects the Pulmonata are intermediate between the 

 Prosobranchs and Opisthobranchs. Like the latter they are 

 orthoneurons and hermaphroditic (p. 376). On the other hand 

 the respiratory organ is far forward near the head, with the result 

 here, as in the Prosobranchs, that the auricle is forward, the aorta 

 behind. The opisthopneumous Testacellidae have the lungs at 

 the posterior end of the body. Here and there streptoneurous 

 conditions occur (Cliilina). 



The lung, the most characteristic feature of the order, is a 

 spacious sac arising from the mantle cavity along with the degen- 

 eration of the ctenidium. It begins on the right side and like a 

 half-moon stretches some distance on the left. On the right side 

 is a small opening, the spiracle, with a sphincter muscle, and in its 

 margin the anus and sometimes the ureter. The roof of the lung 

 is occupied by a rich network of blood vessels (fig. 371, lu) which 

 draw the blood from a marginal vein, collect 

 it in a main trunk and carry it to the heart. 



Many pulmonates are aquatic, but since they have 

 no gills they must occasionally come to the surface to 

 fill the lung sac with air. This is the case with many 

 pond snails of the Limnseidse, but some, which live 

 at great depths, as in the lakes of Geneva and Con- 

 stance, and consequently cannot reach the surface, 

 use the skin and to some extent the lung for 

 water-breathing. Several genera (Planorbis, Pulmo- 

 branchia, Siphonaria) have given rise to secondary 

 gills. 



Sub Order I. STYLOMMATOPHORA. Four re- 

 tractile tentacles, the eyes being borne at the tips of 

 the second and longer pair. The HELICIDJS have a 

 well-developed shell, closed by an epiphragm (p. 372) 

 during hibernation. Helix,* many hundred species 

 distributed among many sub genera. Pupa,* Acha- 

 tina, Bulimus, many tropical species. LIMACID^E. 

 Shell reduced, completely concealed in the mantle. 

 Limax* Avion* Ariolimax.* 



Sub Order II. BASSOMATOPHORA. Only one pair of non-retractile- 

 tentacles, the eyes at their base. LIMN.EID.E, pond snails, living in shallow 

 ponds and brooks. Limncea* Planorbis.* 



FIG. 380. Limax cine- 

 reus. (After Ludwig- 

 Leunis.) s. spiracle. 



