In certain parts of the desert of Algeria, where there is not a trace of vegetation to be seen, and the temperature at mid-day is 110° F., the ground is sometimes so covered with Helix lactea as to appear perfectly white. Dr. F. H. H. Guillemard has told me that he noticed, in somewhat similar surroundings between Fez and Tangier, H. pisana in such extraordinary abundance that they hung from the low scrub in bunches the size of a man’s two fists. It is singular that Mollusca should live, and not only live, but flourish, in localities apparently so unpromising. Shells which occur in the Algerian Sahara are actually larger and altogether finer than the ordinary European form of the same species. In order to protect themselves to some extent against the scorching heat and consequent evaporation, desert species are frequently modified in one of two ways; the shell becomes either white or a light dusky brown, as in the familiar Helix desertorum, or else it gains immensely in thickness. Specimens of H. pomatia, recently procured from Fez, are of extraordinary thickness as compared with forms from our own chalk downs of Kent and Surrey.
Fresh-water Mollusca are frequently found inhabiting hot springs. Thus Neritina fluviatilis lives at Bagnères de Bigorre in water at about 68° F. In another hot spring in the eastern Pyrenees a Bithynia lives at a temperature of over 73° F.; while Blainville mentions another case of a Bithynia living in water at 122° F.
Hibernation and Aestivation.—As autumn begins to draw on, and the first frosts to nip vegetation, terrestrial species retire beneath stones, into cracks in old walls, holes in tree trunks, deep fissures in rocks, and nooks and crannies of every kind, or else bury themselves deeply in the earth or in moss and heaps of leaves. They thus commence their period of hibernation, which varies in length according to the duration of winter. Frequently masses of Helices may be found attached to one another, probably not so much for the sake of warmth, for their temperature is but low, as to share the comforts of a cosy retreat in common. Slugs generally hibernate alone, excavating a sort of nest in the earth, in which they encyst themselves, contracting their bodies until they are almost round, and secreting a covering of their own slime. The Helices usually close up the mouth of their shell by the formation of a membranous or chalky epiphragm, which will be further described below. Both snails and slugs take care to be in good condition at the time their winter sleep begins, and for this reason the former are said to be most esteemed by foreign epicures if captured just at this period.[38]
During hibernation, the action of the heart in land Pulmonata ceases almost entirely. This appears to be directly due to the effect of cold. Mr. C. Ashford has related[39] some interesting experiments made upon H. hortensis and Hyal. cellaria, with the view of ascertaining the effect of cold upon their pulsations. His observations may be tabulated as follows:—
| Number of pulsations per minute | ||
| Helix hortensis | Hyal. cellaria | At degrees Fahr. |
| 22 | 21 | 52° |
| 14 | 12 | 44° |
| 10 | 11 | 38° |
| 4 | 9 | 30° |
At low temperatures the character, as well as the number of the pulsations changed; they became imperfect and intermittent, although exceptionally at 31° F. a H. rufescens gave five or six pulsations a minute, very full and deliberate. The result of taking the Hyalinia suddenly into the heat of a greenhouse was to bring on palpitations. Further experiments resulted in evidence of a similar kind. Hyal. radiatula, placed upon a deal table in a room, showed 52 pulsations per minute at 62° F. Placed upon the palm of the hand, the action soon rose to 108. Hyal. alliaria, similarly treated, rose from 72 pulsations to 110. Floated upon water, the action of the heart of the latter suddenly fell to 29.
Fresh-water Pulmonata do not appear to hibernate. Unio and Anodonta, however, bury themselves more deeply in the mud, and Dreissensia casts off its byssus and retires under the mud in deeper water.[40] Limnaea and Planorbis have often been noticed to crawl about under the lower surface of a thick coating of ice. In periods of prolonged drought, when the water in the ponds dries up, the majority of genera bury themselves in the mud. I have known Limnaea peregra bury itself three inches deep, when surprised by a sudden fall of the water in the ditch on Coe Fen, behind Peterhouse, Cambridge. Physa hypnorum frequents by preference ditches which dry up in summer, as does also Planorbis spirorbis, the latter often forming a sort of epiphragm against evaporation. Ancylus has been observed to spend the whole winter out of water, and P. spirorbis has been noticed alive after four months’ desiccation.[41]
True aestivation, however, occurs mainly in the tropics, where there is no winter, but only a period when it is not quite so hot as the rest of the year, or on a coast like the Mediterranean, which is subject to sudden and severe heat. This period is usually rainless, and the heat is therefore a dry heat. At this season, which may last for three or four months, most of the land Mollusca enter upon a period of inaction, either burying themselves deeply in the ground, or else permanently attaching themselves to the stalks of grass and other herbage, or the under sides of rocks. For instance, the large and beautifully painted Orthalicus, Corona, and Porphyrobaphe, which inhabit Brazil, Ecuador, and eastern Peru, bury themselves deeply in the ground during the dry season, while in the rains they climb to the topmost branches of the great forest trees.[42] Thus it may well happen that a visitor to a tropical island, Ceylon for instance, or one of the Greater Antilles, if he times his visit to coincide with the rainless season, may be grievously disappointed at what seems its unaccountable poverty in land Mollusca. But as soon as the weather breaks, and the moisture penetrates their retreats, every bush and every stone, in favoured localities, will be alive with interesting species.
The Epiphragm.—A considerable number of the land Pulmonata (and a very few of the fresh-water) possess the power of closing the aperture of their shell by means of what is known as an epiphragm or covering of hardened mucus. This epiphragm is habitually formed by certain species during hibernation or aestivation, or even during shorter periods of inactivity and retirement, the object being, either to check evaporation of the moisture of the body, or to secure the animal against the cold by retaining a thin layer of slightly warm air immediately within the aperture of the shell.
The epiphragm differs widely in character in different species, sometimes (Clausilia, Pupa, Planorbis) consisting of the merest pellicle of transparent membrane, while at others (Helix aperta, H. pomatia) it is a thick chalky substance, with a considerable admixture of carbonate of lime, with the consistency of a hardened layer of plaster of Paris. Within these extremes every variety of thickness, solidity, and transparency occurs. During long hibernation several epiphragms are not unfrequently formed by the same individual snail, one within the other, at gradually lessening distances. The epiphragm thus performs, to a certain extent, the part of an operculum, but it must be remembered that it differs radically from an operculum physiologically, in being only a temporary secretion, while the operculum is actually a living part of the animal.