. A manual of zoology. Zoology. IV. GASTEROPODA: PULMONATA. 583 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 otlier 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 Testacellidse have the lungs at the posterior end of the body. Here and there streptoneurous conditions occur (Ckilma). The lung, the most characteristic feature of th
. A manual of zoology. Zoology. IV. GASTEROPODA: PULMONATA. 583 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 otlier 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 Testacellidse have the lungs at the posterior end of the body. Here and there streptoneurous conditions occur (Ckilma). 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 tlie left. On the right side is a small opening, the spiracle, with a sj^hincter 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. ^^0 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 tliey 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 Limnseidie, 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 king for water-breathing. Several genevn (Fkinorbis, Fulmu- bi'anchia, iSijj/ioiiarta) have given rise to secondary gills. Sub Order I. STYLOMMATOPHORA. Four re- tractile tentacles, the eyes being borne at tlie tips of the second and longer pair. The HeliciDjE have a well-developed shell, closed by an epiphragm (p. .373) during hibernation. Helix * many hundred species distributed among many sub genera. Pupa* Acha- tUia. Bulimus, many tropical species. Shell reduced, completely concealed in the mantle. Limax* Avion * Ariolimax^
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