By placing the egg on a looking-glass the act of exclusion can be perfectly observed. For several days the inmate can be seen in motion, until at last a small crack appears in the surface of the shell: this gradually enlarges, until the baby slug is able to crawl out, although it not unfrequently backs into the shell again, as if unwilling to risk itself in the world. When it once begins to crawl freely, it buries itself in the ground for 4 or 5 days without food, after which time it emerges, nearly double its original size. At exclusion, the average length is 9 mm., increasing to 56 mm. after the end of 5 months. Full growth is attained about the middle of the second year, and nearly all die at the end of this year or the beginning of the next. Death from exhaustion frequently occurs after parturition. Death from suffocation is sometimes the result of the formation of small blisters on the margin of the respiratory aperture. The attacks of an internal parasite cause death in a singular way. The upper tentacles swell at the base in such a way as to prevent their extrusion; digestive troubles follow, with rigidity and loss of moisture, and death ensues in 2 or 3 days.
Mr. Wotton isolated newly-hatched specimens, with the view of experimenting on their power of self-fertilisation, if the opportunity of fertilising and being fertilised by others was denied them. One of these, after remaining in absolute solitude for 10½ months, began to lay, scantily at first (11th January, 2; 25th January, 2; 11th February, 2), but more abundantly afterwards (3rd April, 60; 15th and 16th, 70; 29th, 53, etc.), the eggs being hatched out in 42–48 days. The precautions taken seem to have been absolutely satisfactory, and the fact of the power of self-fertilisation appears established as far as Arion ater is concerned.
Braun took young individuals of Limnaea auricularia on the day they were hatched out, and placed them singly in separate vessels with differing amounts of water. This was on 15th June 1887. In August 1888 specimen A had only produced a little spawn, out of which three young were hatched; specimen B had produced four pieces of spawn of different sizes, all of which were hatched; specimen C, which happened to be living with three Planorbis, produced five pieces of spawn distinctly Limnaeidan, but nothing is recorded of their hatching. Self-impregnation, therefore, with a fruitful result, appears established for this species of Limnaea.[97]
Reproduction of Lost Parts.—When deprived of their tentacles, eyes, or portions of the foot, Mollusca do not seem to suffer severely, and generally reproduce the lost parts in a short time. If, however, one of the ganglia is injured, they perish. Certain of the Mollusca possess the curious property of being able to amputate certain parts at will. When Prophysaon, a species of Californian slug, is annoyed by being handled, an indented line appears at a point about two-thirds of the length from the head, the line deepens, and eventually the tail is shaken completely off. Sometimes the Prophysaon only threatens this spontaneous dismemberment; this line appears (always exactly in the same place), but it thinks better of it, and the indentation proceeds no further.[98] According to Gundlach,[99] Helix imperator and H. crenilabris, two large species from Cuba, possess the same property, which is said to be also characteristic of the sub-genus Stenopus (W. Indies). Amongst marine species, Harpa ventricosa and Solen siliqua have been observed to act in a similar way, Harpa apparently cutting off the end of the foot by pressure of the shell. Karl Semper, in commenting on the same property in species of Helicarion from the Philippines (which whisk their tail up and down with almost convulsive rapidity, until it drops off), considers[100] it greatly to the advantage of the mollusc, since any predacious bird which attempted to seize it, but only secured a fragment of tail, would probably be discouraged from a second attack, especially as the Helicarion would meanwhile have had time to conceal itself among the foliage.
Strength and Muscular Force.—The muscular strength of snails is surprisingly great. Sandford relates[101] an experiment on a Helix aspersa, weighing ¼ oz. He found it could drag vertically a weight of 2¼ oz., or nine times its own weight. Another snail, weighing ⅓ oz., was able to drag in a horizontal direction along a smooth table twelve reels of cotton, a pair of scissors, a screwdriver, a key, and a knife, weighing in all no less than 17 oz., or more than fifty times its own weight. This latter experiment was much the same as asking a man of 12 stone to pull a load of over 3¾ tons.
If a snail be placed on a piece of glass and made to crawl, it will be seen that a series of waves appear to pursue one another along the under surface of the foot, travelling from back to front in the direction in which the animal is moving. Simroth has shown that the sole of the foot is covered with a dense network of muscular fibres, those which run longitudinally being chiefly instrumental in producing the undulatory motion. By means of these muscles the sole is first elongated in front, and then shortened behind to an equal extent. Thus a snail slides, not on the ground, but on its own mucus, which it deposits mechanically, and which serves the purpose of lubricating the ground on which it travels. It has been calculated that an averaged sized snail of moderate pace progresses at the rate of about a mile in 16 days 14 hours.[102]
Sudden Appearance of Mollusca.—It is very remarkable to notice how suddenly Pulmonata seem to appear in certain districts where they have not been noticed before. This sudden appearance is more common in the case of fresh-water than of land Mollusca, and there can be little doubt that, wherever a new pond happens to be formed, unless there is something in its situation or nature which is absolutely hostile to molluscan life, Mollusca are certain to be found in it sooner or later. “Some 23 years ago,” writes Mr. W. Nelson,[103] “I was in the habit of collecting shells in a small pond near to the Black Hills, Leeds. At that time the only molluscan forms found there were a dwarf form of Sphaerium lacustre, Pisidium pusillum, Planorbis nautileus, and Limnaea peregra. About 10 years ago I resumed my visits to the locality, and found, in addition to the species already enumerated, Planorbis corneus. These were the only species found there until this spring [1883], when, during one of my frequent visits, I was surprised to find Physa fontinalis and Planorbis vortex were added to the growing list of species. Later on Pl. carinatus, Limnaea stagnalis, and Ancylus lacustris turned up; and during June, Pl. contortus was found in this small but prolific pond.” Limnaea glutinosa is prominent for these remarkable appearances and disappearances. In 1822 this species suddenly appeared in some small gravel pits at Bottisham, Cambs., in such numbers that they might have been scooped out by handfuls. After that year they did not appear numerous, and after three or four seasons they gradually disappeared.[104] Physa (Aplecta) hypnorum is noted in a similar way. In February 1852, for instance, after a wet month, the water stood in small puddles about 3 feet by 2 in a particular part of Bottisham Park which was sometimes a little swampy, though usually quite dry. One of these puddles was found to contain immense numbers of the Aplecta, which up to that time had not been noted as occurring in Cambridgeshire at all.[105] In a few days the species entirely disappeared and was never again noticed in the locality.[106]
Writing to the Zoological Society of London from New Caledonia, Mr. E. L. Layard remarks:[107] “The West Indian species Stenogyra octona has suddenly turned up here in thousands; how introduced, none can tell. They are on a coffee estate at Kanala on the east coast. I have made inquiries, and cannot find that the planter ever had seed coffee from the West Indies. All he planted came from Bombay, and it would be interesting to find out whether the species has appeared there also.”
Sometimes a very small event is sufficient to disturb the natural equilibrium of a locality, and to become the cause either of the introduction or of the destruction of a species. In 1883 a colony of Helix sericea occupied a portion of a hedge bottom twenty yards long near Newark. It scarcely occurred outside this limit, but within it was very plentiful, living in company with H. nemoralis, H. hortensis, H. hispida, H. rotundata, Hyalinia cellaria and Hy. nitidula, and Cochlicopa lubrica. In 1888 the hedge was well trimmed, but the bottom was not touched, and the next year a long and careful search was required to find even six specimens of the sericea.[108]
Showers of Shells.—Helix virgata, H. caperata, and Cochlicella acuta sometimes occur on downs near our sea-coasts in such extraordinary profusion, that their sudden appearance out of their hiding-places at the roots of the herbage after a shower of rain has led to the belief, amongst credulous people, that they have actually descended with the rain. There seems, however, no reason to doubt that Mollusca may be caught up by whirlwinds into the air and subsequently deposited at some considerable distance from their original habitat, in the same way as frogs and fishes. A very recent instance of such a phenomenon occurred[109] at Paderborn, in Westphalia, where, on 9th August 1892, a yellowish cloud suddenly attracted attention from its colour and the rapidity of its motion. In a few moments it burst, with thunder and a torrential rain, and immediately afterwards the pavements were found to be covered with numbers of Anodonta anatina, all of which had the shell broken by the violence of the fall. It was clearly established that the shells could not have been washed into the streets from any adjacent river or pond, and their true origin was probably indicated when it was found that the funnel-shaped cloud which burst over the town had passed across the one piece of water near Paderborn, which was known to contain the Anodonta in abundance.