Broadly speaking, Mollusca whose usual habitat is the water ‘breathe’ water, while those whose usual habitat is the land ‘breathe’ air. But this rule has its exceptions on both sides. The great majority of the fresh-water Mollusca which are not provided with an operculum (e.g. Limnaea, Physa, Planorbis), breathe air, in spite of living in the water. They make periodic visits to the surface, and take down a bubble of air, returning again for another when it is exhausted. On the other hand many marine Mollusca which live between tide-marks (e.g. Patella, Littorina, Purpura, many species of Cerithium, Planaxis, and Nerita) are left out of the water, through the bi-diurnal recess of the tide, for many hours together. Such species invariably retain several drops of water in their branchiae, and, aided by the moisture of the air, contrive to support life until the water returns to them. Some species of Littorina (e.g. our own L. rudis and many tropical species) live so near high-water mark that at neap-tides it must frequently happen that they are untouched by the sea for several weeks together, while they are frequently exposed to a burning sun, which beats upon the rocks to which they cling. In this case it appears that the respiratory organs will perform their functions if they can manage to retain an extremely small amount of moisture.[265]
The important part which the respiratory organs play in the economy of the Mollusca may be judged from the fact that the primary subdivision of the Cephalopoda into Dibranchiata and Tetrabranchiata is based upon the number of branchiae they possess. Further, the three great divisions of the Gasteropoda have been named from the position or character of the breathing apparatus, viz. Prosobranchiata, Opisthobranchiata and Pulmonata, while the name Pelecypoda has hardly yet dispossessed Lamellibranchiata, the more familiar name of the bivalves.
Respiration may be conducted by means of—(a) Branchiae or Gills, (b) a Lung or Lung-cavity, (c) the outer skin.
In the Pelecypoda, Cephalopoda, Scaphopoda, and the great majority of the Gasteropoda, respiration is by means of branchiae, also known as ctenidia[266], when they represent the primitive Molluscan gill and are not ‘secondary’ branchiae (pp. [156], [159]).
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.
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.
Fig. 58.—Aeolis despecta Johnst., British coasts. (After Alder and Hancock.)