In all non-operculate land and fresh-water Mollusca, in the Auriculidae, and in one aberrant operculate (_Amphibola_), respiration is conducted by means of a lung-cavity, or rarely by a true lung, whence the name _Pulmonata_. The land operculates (Cyclophoridae, Cyclostomatidae, Aciculidae, and Helicinidae) also breathe air, but are not classified as Pulmonata, since other points in their organisation relate them more closely to the marine Prosobranchiata. Both methods of respiration are united in _Ampullaria_, which breathes indifferently air through a long siphon which it can elevate above the surface of the water, and water through a branchia (see p. 158). _Siphonaria_ (Fig. 57) is also furnished with a lung-cavity as well as a branchia. Both these genera may be regarded as in process of change from an aqueous to a terrestrial life, and in _Siphonaria_ the branchia is to a great extent atrophied, since the animal is out of the water, on the average, twenty-two hours out of the twenty-four. In the allied genus _Gadinia_, where there is no trace of a branchia, but only a lung-cavity, and in _Cerithidea obtusa_, which has a pulmonary organisation exactly analogous to that of _Cyclophorus_,[267] this process may be regarded as practically completed. [Illustration: FIG. 57.--=A=, _Siphonaria gigas_ Sowb., Panama, the animal contracted in spirit: _gr_, siphonal groove on right side. =B=, _Gadinia peruviana_, Sowb., Chili, shell only: _gr_, mark of siphonal groove to right of head.] Respiration by means of the skin, without the development of any special organ, is the simplest method of breathing which occurs in the Mollusca. In certain cases, _e.g._ _Elysia_, _Limapontia_, and _Cenia_ among the Nudibranchs, and the parasitic _Entoconcha_ and _Entocolax_, none of which possess breathing organs of any kind, the whole outer surface of the body appears to perform respiratory functions. In others, the dorsal surface is covered with papillae of varied size and number, which communicate with the heart by an elaborate system of veins. This is the case with the greater number of the _Aeolididae_ (Fig. 58, compare Fig. 5, C), but it is curious that when the animal is entirely deprived of these papillae, respiration appears to be carried on without interruption through the skin. [Illustration: FIG. 58.--_Aeolis despecta_ Johnst., British coasts. (After Alder and Hancock.)]