It seems probable, as has been already shown (pp. 18–22), that all Pulmonata are ultimately derived from marine forms which breathed water by means of branchiae. Thus we find intermediate forms, such as _Siphonaria_, possessed of both a branchia and a pulmonary sac, the former being evanescent, while in _Gadinia_ and _Amphibola_ it has quite disappeared. In the vast majority of _Pulmonata_ no trace of a branchia remains; its function is performed by a chamber, always situated at the right side of the animal, and generally more or less anterior, admitting air by a narrow aperture which is rhythmically opened and closed. In _Arion_ and _Geomalacus_ (Fig. 69) this aperture is in the front of the right side of the ‘shield,’ in _Limax_ (Fig. 71) in the hinder part, in _Testacella_ (Fig. 20) it is near the extremity of the tail, under the spire of the shell; in _Janella_ it is on the middle of the right edge of the shield (Fig. 70). If a specimen of _Helix aspersa_, or better, of _H. pomatia_, is held up to the light, the beautiful arborescent vessels, with which the upper part of the pulmonary chamber is furnished, can be clearly seen by looking through the aperture as it dilates. It is only in the Auriculidae that an actual spongy mass of lung material appears to exist. When in motion, a _Helix_ inspires air much more frequently than when at rest. Temperature, too, seems to affect the number of inspirations; it appears doubtful whether, during hibernation, a snail breathes at all. In any case, the amount of air required to sustain life must be small. [Illustration: FIG. 70.--_Janella hirudo_ Fisch., N. Caledonia: =G=, generative orifice; =P=, pulmonary orifice; =T=, =T=, tentacles. (After Fischer.)] [Illustration: FIG. 71.--_Limax maximus_ L.: =PO=, pulmonary orifice. × ⅔.]