Fig. 71.—Limax maximus L.: PO, pulmonary orifice. × ⅔.
With regard to the respiration of fresh-water Pulmonata there appears to be some difference of opinion. It is held, on the one hand, that the Limnaeidae only respire air, making periodic visits to the surface to procure it, and that they perish, if prevented from doing so, by asphyxiation. If, we are told,[270] as a Limnaea is floating on the surface of the water in a glass jar, a morsel of common salt be dropped upon its outstretched foot, it will sink heavily to the bottom, emitting a stream of air from its pulmonary orifice. On recovering from the shock, it will anxiously endeavour to regain the surface, but will have some difficulty in doing so, owing to its now much greater specific gravity. When it succeeds, it creeps almost out of the water, and exposes its respiratory orifice freely to the air. If the experiment is repeated several times on the same individual, it becomes so much weakened that it has to be taken out of the water to save its life. Moquin-Tandon, on the other hand, is strongly of opinion[271] that there is no absolute necessity for Limnaea to obtain air by rising to the surface, and that, if prevented from emerging, it can obtain air from the water. When covered in by a roof of ice, Limnaea has not been observed to suffer any inconvenience. Moquin-Tandon kept L. glabra and Planorbis rotundatus in good health under 20 mm. of water for eighteen and nineteen days, and relates a case in which Physa was kept alive under water for four days, and Planorbis for twelve. Young specimens, both of Limnaea and Planorbis, do not rise to the surface for a supply of air; they are hatched with the pulmonary cavity full of water.
It is probable, therefore, that Limnaeidae are capable, on occasion, of respiration through the skin. Some authorities are of opinion that certain long and narrow lamellae, situated within the pulmonary sac, are employed for the purpose of aqueous respiration. Ancylus, which never makes periodic excursions to the surface, perhaps respires by receiving into its pulmonary chamber the minute quantities of oxygen given off by the vegetation on which it feeds.
Limnaeidae taken from a great depth of water, e.g. from 130 fathoms in the lake of Geneva, have been examined by Forel.[272] The pulmonary sac is full of water, but there is no transformation of organs, no appearance of a branchia, to meet the changed circumstances of their environment. Doubtless a good deal of respiration is done by the skin; being soft and vascular, it respires the air dissolved in the water. Forel cites cases of Limnaea living at much shallower depths, which come to the surface once, and then remain below for months. The oxygen of this supply must soon have become exhausted, and the animals, discontinuing for a time the use of the pulmonary chamber, must have respired through the skin. Shallow-water Limnaea, according to the same authority, remain beneath the surface during cold weather; when warm weather returns they rise to the surface to take in a supply of air. Since the water at great depths is always very cold, there is no need for the Limnaea living there to rise to the surface at all.
It is a curious fact that Limnaea, which have been respiring by the skin for the whole winter, should suddenly, on the first warm days of summer, take to rising to the surface and breathing air. But exactly the same phenomenon is shown in the case of Limnaea from great depths. Placed in an aquarium, they immediately begin rising to the surface and inspiring air; in other words, they experience instantaneously a complete transformation of their respiratory system.
In Onchidium, a land pulmonate which has retrogressed to an amphibious or quasi-marine mode of life, there is no organ which represents the pulmonary or branchial cavity, the so-called lung being only a cavity of the kidney. Respiration is, however, conducted by the skin as well, and by the dorsal papillae.[273]
Land Mollusca can sustain, for a considerable time, complete deprivation of atmospheric air. Helices placed in an exhausted receiver show no signs of being inconvenienced for about 20 hours, and are able to survive for about two or three days. If detained under water, they are very active for about 6 hours, then become motionless, the body swells, owing to the water absorbed, and death ensues in about 36 hours. Immersion for only 24 hours is generally followed by recovery. In the latter case, the cause of death is not so much deprivation of air as compulsory absorption of water by the skin. The amount of water thus taken up is surprising. Spallanzani found that a Helix which weighed 18 grammes increased in weight by 13½ grammes after a prolonged immersion. Even slugs enclosed in moist paper gained more than 2 grammes in the course of half an hour. Experiment has shown that the amount of carbonic acid gas produced by respiration stands in direct relation to the amount of food consumed. Four pairs of snails were taken which had recently awakened from their winter sleep and had eaten heartily, and an equal number, under the same circumstances, which had been prevented from eating. It was found that the first four pairs produced, in consuming a given amount of oxygen, 11, 9, 10, and 13 parts respectively of carbonic acid, while the second set produced, in consuming the same amount of oxygen, only 4, 8, 7, and 9 parts of carbonic acid.[274] Hibernating Helices, if weighed in December and again in April, will be found to have lost weight, due to the expiration of carbonic acid. Owing to the difficulty of experiment, opinions vary as to the absolute temperature of snails. It appears to be established that several snails, if placed together in a tube, raise the temperature one or two degrees C., but as a rule, the temperature of a solitary Helix differs very slightly from that of the surrounding air. Increased activity, whether in respiration or feeding, is found to raise the temperature.
Fig. 72.—Cardium edule L.: A, anal; B, branchial siphon; F, foot. (After Möbius.)
W. H. Dall, writing of the branchia in Pelecypoda, remarks[275] that there can be no doubt that its original form was a simple pinched-up lamella or fold of the skin or mantle. This, elongated, becomes a filament. Filaments united by suitable tissue, trussed, propped, and stayed by a chitinous skeleton, result in the forms, wonderful in number and complexity, which puzzle the student to describe, much more to classify.