Fig. 165.—Neritina longispina Récl., Mauritius. (Operculum removed.)
The various details of sculpture on the exterior surface of the shell, the striae, ribs, nodules, imbrications, spines, and other forms of ornamentation are all the product of similar and corresponding irregularities in the mantle margin, and have all been originally situated at the edge of the lip. Spines, e.g. those of Murex and Pteroceras, are first formed as a hollow thorn, cleft down its lower side, and are afterwards filled in with solid matter as the mantle edge withdraws. What purpose is served by the extreme elaboration of these spiny processes in some cases, can hardly be considered as satisfactorily ascertained. Possibly they are a form of sculptural development which is, in the main, protective, and secures to its owners immunity from the attacks of predatory fishes.
‘Attached’ genera (e.g. Chama, Spondylus) when living on smooth surfaces have a flat shell, but when affixed to coral and other uneven surfaces they become very irregular in shape. The sculpture of the base on which they rest is often reproduced in these ‘attached’ shells, not only on the lower, but also on the upper valve, the growing edge of which rests on the uneven surface of the base. Oysters attached to the branches of the mangrove frequently display a central convex rib, modelled on the shape of the branch, from which the plaits of sculpture radiate, while specimens fixed to the smooth trunk have no such rib. Crepidula, a genus which is in the habit of attaching itself to other shells, varies in sculpture according to that of its host. Sometimes the fact may be detected that a specimen has lived on a ribbed shell when young, and on a smooth one when old, or vice versâ. A new genus was actually founded by Brown for a Capulus which had acquired ribs through adhesion to a Pecten. A specimen of Hinnites giganteus in the British Museum must at one period of its growth have adhered to a surface on which was a Serpula, the impression of which is plainly reproduced on the upper valve of the Hinnites.[339]
Fig. 166.—A specimen of Anomia ephippium L., Weymouth, taken upon Pecten maximus, the sculpture of which is reproduced on the upper valve of the Anomia, and even on a young Anomia attached to the larger specimen.
Growth of the Shell.—Nothing very definite is known with regard to the rate of growth of the shell in marine Mollusca. Under favourable conditions, however, certain species are known to increase very rapidly, especially if the food supply be abundant, and if there is no inconvenient crowding of individuals. Petit de la Saussaye mentions[340] the case of a ship which sailed from Marseilles for the west coast of Africa, after being fitted with an entirely new bottom. On arriving at its destination, the vessel spent 68 days in the Gambia River, and took 86 days on its homeward voyage. On being cleaned immediately on its return to Marseilles, an Avicula 78 mm. and an Ostrea 95 mm. long (both being species peculiar to W. Africa) were taken from its keel. These specimens had therefore attained this growth in at most 154 days, for at the period of their first attachment they are known to be exceedingly minute. P. Fischer relates[341] that in 1862 a buoy, newly cleaned and painted, was placed in the basin at Arcachon. In less than a year after, it was found to be covered with thousands of very large Mytilus edulis, 100 mm. × 48 mm., the ordinary size on the adjoining banks being only about 50 to 60 × 30 mm.
Some observations have already been recorded (p. [40]) on the growth of Helix aspersa. In the summer of 1858, which was very dry, especially in the south of France, the young Helices born that year were still very small in August. About the end of that month abundant rain came on, and in four or five days young H. variabilis, H. pisana, and H. aspersa, eating without cessation, as if to make up for lost time, grew more than a centimetre of shell. The lip of a young H. arbustorum has been observed to have grown, at the end of the first week in the season’s growth, 3 mm., at the end of the second week, 6·25 mm., the third, 11·5 mm., and the fourth 12·5 mm., with a finished lip.[342]
Careful observation has shown that in the growth of the shell of Helix aspersa the periostracum is first produced; it is covered with hyaline globules, 10–12 mm. in diameter, which persist even in the oldest shells. Calcareous matter is deposited on the internal face of the new periostracum, at some distance from the margin. It is secreted by a white zone or band of cells bounding the entire breadth of the mantle as applied to the peristome. Immediately behind the white zone are a series of pigment cells which not only give the shell its colour but complete the calcification of the shelly matter laid down by the white zone. When the animal has attained its full growth and the lip is finished off, the white band and the periostracum cells completely disappear, and only such cells persist as contribute to the internal thickening of the shell. Shell growth, in this species, is very rapid. If a portion of the pulmonary sac is laid bare, by removing a fragment of shell, at the end of 1½ or 2 hours there may be detected a delicate organic membrane covering the hole, and strewn with crystals of carbonate of lime. This thickens with great rapidity, and soon fills up the hole with solid matter. For two consecutive months an animal, deprived of food, has been known to reproduce this membrane daily after its removal every morning.[343] Prof. Schiedt has found that oysters, if deprived of the right valve and exposed to the light, not only develop brown pigment over the whole exposed surface of mantle and branchiae, but actually succeed in part in reproducing the valve and hinge.[344]
Deposit of Additional Layers of Shell.—Mollusca possess the power of thickening the interior of the shell, by the deposit of successive layers. This is frequently done in self-defence against the attacks of boring Mollusca, sponges, and worms. Cases may often be noticed of Ostrea, Spondylus, and other sedentary molluscs, which, unable to escape the gradual assaults of their foes, have provided against them by the deposit of fresh shelly matter. A somewhat similar plan is adopted to provide against intrusion by way of the aperture. Pearls are, in many cases, the result of shell deposition upon the eggs or even the body of some intrusive parasite (Distoma, Filaria, etc.), and are, in some countries, artificially produced by the introduction of fragments of sand, metal, etc., into living Unio and Anodonta. Little joss images are made in India and China, the nacre on which is produced by thrusting them inside living Unionidae.