(4) Most species bury themselves in the ground to increase the dimensions of their shells.
Six recently hatched H. pomatia were placed in a box and regularly fed on lettuce and cabbage leaves from August until December, when they buried themselves in the soil for winter; at this period they had gradually increased in dimensions to the size of H. hispida. On the 1st April following, the box was placed in the garden, and on the 3rd the Helices reappeared on the surface, being no larger in size than they were in December. Although regularly fed up to 20th June, they were not perceptibly larger, but on that day five of them disappeared, having buried themselves, with the mouth of the shell downwards, in the soil. After ten days they reappeared, having in that short time grown so rapidly as to be equal in size to H. pisana. On the 15th July they again buried themselves, and reappeared on 1st August, having again increased in size. For three months from this date they did not become perceptibly larger; on 2nd November food was withheld for the winter and they became dormant.
A similar experiment, with similar results, was carried on with a number of H. aspersa, hatched on 20th June. During the summer they grew but little, buried themselves on 10th October with the head upwards, and rose to the surface again on 5th April, not having grown during the winter. In May they buried themselves with the head downwards, and appeared again in a week double the size; this went on at about fortnightly intervals until 18th July, when they were almost fully grown.
Helix nemoralis, H. virgata, H. caperata, and H. hispida bury themselves to grow; H. rotundata burrows into decayed wood; Hyalinia radiatula appears to remain on decaying blades of grass; Pupa umbilicata, Clausilia rugosa, and Buliminus obscurus bury their heads only.
The observations of Mr. W. E. Collinge[90] do not at all agree with those of Mr. Lowe, with regard to the mode in which land Mollusca enlarge their shells. He bred and reared most of the commoner forms of Helix and also Clausilia rugosa, but never saw them bury any part of their shell when enlarging it. While admitting that they may increase their shells when in holes or burrows of earthworms, he thinks that the process of burying would seriously interfere with the action of the mantle during deposition, and in many cases damage the membranaceous film before the calcareous portion was deposited. Mr. Collinge has found the following species under the surface in winter: Arion ater (3–4 in.), Agriolimax agrestis, (6–8 in.), Hyalinia cellaria and H. alliaria (6–8 in.), Hyalinia glabra (5 in.), Helix aspersa (5–6 in.), H. rufescens (4–6 in.), H. rotundata (4–5 in.), H. hispida (7 in.), Buliminus obscurus (4–6 in.), B. montanus[91] (24 in.), and the following in summer, Hyalinia cellaria and alliaria (6–8 in.), Helix rotundata (4–5 in.), Balea perversa (6–8 in.), Cyclostoma elegans (3–4 in.). The same author has found the following species of fresh-water Mollusca living in hard dry mud: Sphaerium corneum (3–14 in.), S. rivicola (5–6 in.), S. lacustre (10–14 in.), all the British species of Pisidium (4–12 in.), Limnaea truncatula (18 in., a single specimen). All our species of Unio, Anodonta, Bithynia, and Paludina bury themselves habitually in fine or thick wet mud, to a depth of from 4 to 14 inches.
This burying propensity on the part of Mollusca has been known to play its part in detecting fraud. When my friend Mr. E. L. Layard was administering justice in Ceylon, a native landowner on a small scale complained to him of the conduct of his neighbour, who had, during his absence from home, diverted a small watercourse, which ran between their holdings, in such a way as to filch a certain portion of the land. The offender had filled up and obliterated the ancient course of the stream, and protested that it had never run but in its present bed. Mr. Layard promptly had a trench sunk across what was said to be the old course, and the discovery of numerous living Ampullaria, buried in the mud, confirmed the story of one of the litigants and confounded the other.[92]
Depositing and Hatching of Eggs: Self-fertilisation.—There appears to be no doubt that Helices, when once impregnated, can lay successive batches of eggs, and possibly can continue laying for several years, without a further act of union. A specimen of Helix aspersa was noticed in company with another on 5th August; on 9th August it laid eggs in the soil, and early in the following summer it laid a second batch of eggs, although its companion had been removed directly after its first introduction. An Arion received from a distance laid 30 eggs on 5th September, and 70 more on the 23rd of the same month, although quite isolated during the whole time.[93] By far the most remarkable case of the kind is related by Gaskoin.[94] A specimen of Helix lactea was kept in a drawer for about two years, and then in another drawer for about two years more. It was then taken out, and placed in water, when it revived, and was placed alone under a bell jar with earth and food. Six months after, about 30 young H. lactea were found crawling on the glass, the act of oviposition not having been observed.
The observations of Mr. F. W. Wotton,[95] with regard to the fertilisation and egg-laying of Arion ater, are of extreme interest and value. A pair of this species, kept in captivity, united on 10th September 1889, the act lasting about 25 minutes. From that date until the eggs were laid, the animals looked sickly, dull of colour, with a somewhat dry skin. Eggs were deposited in batches, one, which we will call A, beginning three days before B. On 10th October A laid 80 eggs; on the 16th, 110; on the 25th, 77; on 8th November, 82; and on 17th November, 47; making a total of 396. Specimen B, which began on 13th October, three days after A, made up for the delay by laying 246 eggs in 40 hours; on 26th October it laid 9, on 10th November, 121; and on 30th November, 101; a total of 477. These eggs weighed 624 to the ounce, and, in excluding the batch of 246, B parted with ⅜ of its own weight in 40 hours, while the whole number laid were rather over ¾ of its own weight!
While depositing the eggs, the slug remained throughout in the same position on the surface of the ground, with the head drawn up underneath the mantle, which was lifted just above the reproductive orifice. When taken into the hand, it went on laying eggs without interruption or agitation of any kind. After it had finished laying it ate half a raw potato and then took a bath, remaining submerged for more than an hour. Bathing is a favourite pastime at all periods. Specimens, says Mr. Wotton, have survived a compulsory bath, with total submersion, of nearly three days’ duration.
Mr. Wotton’s account of the hatching of the eggs is equally interesting. It is noticeable that the eggs of one batch do not hatch by any means simultaneously; several days frequently intervene. The average period is about 60 days, a damp and warm situation bringing out the young in 40 days, while cold and dryness extended the time to 74 days, extremes of any kind proving fatal. Of the batch of eggs laid by B on 30th November, the first 2 were hatched on the following 16th January, and 2 more on the 17th; others, from 10 to 20, followed suit on the succeeding 5 days, until 82 in all were hatched, the remaining 19 being unproductive.[96]