Fig. 68.—Pleurophyllidia lineata Otto, Mediterranean: a, anus; br, secondary branchiae; m, mouth; s.o, sexual orifice.
Certain of the Nudibranchiata possess no special breathing organs, and probably respire through the skin (Elysia, Limapontia, Cenia, Phyllirrhoë). The majority, however, have developed secondary branchiae, in the form of prominent lobes or leaf-like processes (the cerata), which are carried upon the back, without any means of protection. These cerata are, as a rule, of extreme beauty and variety of form, consisting sometimes of long whip-like tentaculae, in other cases of arborescent plumes of fern-like leafage, in others of curious bead-like appendages of every imaginable shape and colour. In Doris they lie at the posterior end of the body, in a sort of rosette, which is generally capable of retraction into a chamber. In Phyllidia and Pleurophyllidia these secondary branchiae lie, as in Patella, on the lateral portions of the mantle.
The Scaphopoda in all probability possess neither true nor secondary branchiae.
Pulmonata.—When we use the term ‘lung,’ it must be remembered that this organ in the Mollusca does not correspond, morphologically, with the spongy, cellular lung of vertebrates; it simply performs the same functions. The ‘lung,’ in the Mollusca, is a pouch or cavity, lined with blood-vessels which are disposed over its vaulted surface in various patterns of network. The pulmonary sac or cavity is therefore a better name by which to denote this organ.
Fig. 69.—Geomalacus maculosus Allm., S. Ireland: P.O, pulmonary orifice.
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.
Fig. 70.—Janella hirudo Fisch., N. Caledonia: G, generative orifice; P, pulmonary orifice; T, T, tentacles. (After Fischer.)
