Fig. 18.—A, Neritina reticularis Sowb., Calcutta (brackish water); B, Helicina neritella Lam., Jamaica (land); C, Proserpina (Ceres) eolina Ducl., Central America (land).
Gasteropoda.—(2) Pulmonata. The origin of these, the bulk of the land fauna, must at present be regarded as a problem not yet finally solved. Some authorities, as we have seen, regard them as derived from the Nudibranchiate, others, probably more correctly, from the Tectibranchiate Opisthobranchs.
The first known members of the land Pulmonata (Pupa [?], Hyalinia) are from the Carboniferous of North America. Similar but new forms appear in the Cretaceous, from which time to the present we have an unbroken series. The characteristically modern forms, according to Simroth,[31] are Helices with thick shells. According to the same author, Vitrina and Hyalinia are ancestral types, which give origin not only to many modern genera with shells, but to many shell-less genera also, e.g. Testacella is probably derived through Daudebardia from Hyalinia, while from Vitrina came Limax and Amalia. A consideration of the radulae of the genera concerned certainly tends in favour of these views.
Godwin-Austen, speaking generally, considers[32] genera of land Pulmonata with strongly developed mantle-lobes and rudimentary shell as more advanced in development than genera in which the shell is large and covers all or nearly all the animal.
CHAPTER II
LAND AND FRESH-WATER MOLLUSCA, THEIR HABITS AND GENERAL ECONOMY
The majority of the Land Mollusca are probably more sensitive than is usually believed. The humidity of the air must affect the surface of their skin to a considerable extent. Every one has noticed how the snails ‘come out’ on a damp evening, especially after rain. As a rule, they wait till rain is over, probably objecting to the patter of the drops upon their delicate tentacles. Snails kept in captivity under a bell-glass are acutely sensitive of a damp atmosphere, and will bestir themselves after rain just as if they were in the open air. Certain Helices which are accustomed to live in moist places, will find their way to water, if removed from their usual haunts. A case is recorded[33] of a specimen of H. arbustorum, kept in a kitchen, which used to find its way directly under the cold water tap, and appeared to enjoy the luxury of a douche. How delicately the conditions of life are balanced in some of these creatures is seen in the case of Omalonyx, a genus akin to Succinea, which is found in Brazil and the northern parts of South America. It lives creeping on plants which overhang the margin of water, but perishes equally, if placed in the water itself, or removed to a distance from it for any length of time.[34]
Endurance of Heat and Cold.—The Mollusca are capable, at least as far as some species are concerned, of enduring severe extremes both of cold and heat. The most northern pulmonate yet observed is a fresh-water species, Physa (Aplecta) hypnorum L. This hardy mollusc, whose shell is so fragile as to need most careful handling, has been noticed on the peninsula of Taimyr, North Siberia, in 73° 30’ N. lat, a region whose mean annual temperature is below 10° F. with a range of from 40° F. in July to -30° F. in January.
It is well known that the Limnaeidae, and probably most fresh-water Mollusca of sub-temperate regions, can continue to live not merely under, but enveloped in ice, and themselves frozen hard. Garnier relates[35] that, during the winter of 1829–30, some large Limnaea auricularia, which had been placed in a small basin, were frozen into a solid mass, experiencing a cold of -2° F. He supposed they were dead, but, to his surprise, when the basin thawed, the Limnaea gradually revived. Paludina vivipara and Anodonta anatina have been known to resist a temperature of 23° F., and the former has produced young shortly after being thawed out of the ice.[36] As far north as Bodø in Norway (67° 37’ N. lat., well within the Arctic circle) there are found no less than fourteen species of terrestrial Mollusca, among them being Balea perversa and Clausilia rugosa.[37]
Vitrina is one of our most hardy molluscs, and may be observed crawling on bright mornings over the frost-covered leaves of a wood or copse. V. glacialis is said by Charpentier to live in the Alps at a height where the stones are covered with snow from nine to ten months of the year. Many of the Hyaliniae are very hardy. Arion, in spite of having no external shell to protect it, is apparently less affected by the cold than Helix, and does not commence hibernation till a later period in the autumn. The operculate land Mollusca, in spite of the protection which their operculum may be supposed to afford, are exceedingly sensitive to cold, and the whole group is without doubt a product of tropical or semi-tropical regions (see map at frontispiece). A species of Helicina which inhabits the southern States of North America has been known to be almost exterminated from certain districts by the occurrence of an unusually severe winter.
One of the highest altitudes at which a land shell is known to live appears to be the Liti Pass (Himalayas, 14,000 ft.). At this enormous altitude, two species of Buliminus (arcuatus Hutt. and nivicola Bens.) live on juniper bushes among patches of snow. An Anadenus is said to have been found in a similar locality at 15,000 ft., while Limnaea Hookeri has been taken from over 16,400 ft. in Landour. In the Andes of Peru and Bolivia, five species of Bulimulus, one of Pupa, and one of Limax occur at an elevation of 10,500 to 15,000 ft. Several fresh-water Mollusca inhabit Lake Titicaca, which stands at a height of 12,550 ft. in the Bolivian table-land.