Donax cuneata.
Donax cuneata.
In following the system of Lamarck, we are gradually led through various genera to two species figured on the next page, namely, theTellina, orearth-shell, and the Pandora, and, ultimately, to the Solen, orrazor-shell.
The Solens are singular from the power they possess of burying themselves in the sands on the coast, sometimes even to the depth of a couple of feet. The foot of the Solen, by means of which it is able to penetrate the sand, is equal to one-half of the length of the shell. Their movements are confined to rising to the surface of the sand inwhich they have formed their hole, and in again sinking to the bottom. This movement is, no doubt, produced by the action of the foot, which forms itself into a sharp point in its descent, and when it remounts is enlarged as much as possible, to form a resting-point, for the purpose of raising the shell to the surface. It is not supposed that the animal ever entirely leaves its hole of its own accord, although it may possess the power; but it is certain, according to the observations of Réaumur, that if forcibly removed it can re-enter it. The hole it forms for its retreat is always perpendicular.
Tellina.Solen vagina.Pandora rostrata.
Tellina.Solen vagina.Pandora rostrata.
One of these creatures, being taken out of its retreat, was laid on the sand; it first extended its foot in the form of a wedge, or rather cone, and, applying it to the surface of the sand, slightly raised the farthest end of its shell; at the next effort the projecting part of the foot was buried in the sand, and the shell became more elevated; after two or three more attempts the hole had attained a perpendicular direction, and the shell was partly buried in it; the shell then began to descend, and that with considerable quickness.
TheSolensare used sometimes, but rarely, for food; but in placeswhere they abound, they are sought after as bait for fishes: the method of taking them is very singular. Having discovered the place of retreat of the creature, by observing the hole in the sand which leads to its chamber, the fisherman throws into its entrance a small quantity of salt. Although an inhabitant of salt water, the pure salt produces so irritating an effect on the extremity of its body, that it quickly mounts to the surface; the fisherman, waiting for its appearance, snatches hastily at it, and if he succeeds in seizing it firmly, makes good his capture; but if not sufficiently active, and the animal escapes, the application of fresh salt produces no further effect; either it is not sensible to the additional infusion of salt, or, which is most likely, the instinct of self-preservation causes it to put up with the inconvenience rather than be taken. In this case, no other means are left of securing it, than using an iron instrument to dig it out with. The number of species is upwards of twenty.
These creatures have much more powerful means of boring than the solens, for not only do they imbed themselves in hard clay, but even in stone and lava. In what manner this is effected has been the subject of much dispute; it is almost certain that the process is not mechanical, for their soft body, and the fragile nature of their shells, seem an insuperable bar to such a proceeding. Some authors have asserted that the hole is formed by means of an acid secreted by the animal, which acts chemically on the stone; but there are two reasons against this solution of the difficulty; first, no acid liquid has been discoveredin the living animal, and, secondly, although acid would act upon stone, it would have no effect whatever on lava.
Another curious part of the history of these shell-fish is their phosphorescence, which is so bright, that it has been asserted, if eaten in the dark without their being cooked, it appears as if the person devouring them was swallowing phosphorus.
Pholas dactyloides.
Pholas dactyloides.
Although not used as food in this country, they are not uncommonly eaten on the shores of the Mediterranean, where some large species are found.
The species represented in the engraving is found on the British coasts, imbedded in clay; its shell is of a delicate white, beautifully carved. The projecting piece at the lower part of the shell in the engraving is the long foot of the animal; on the right hand, the hole from which a shell has been removed is shown.
Before the anatomy of this destructive creature had been carefully examined, there was as much difficulty in guessing at the means it employed in penetrating the solid timbers in which it is found, as there is with regard to the operations of the Pholas; but subsequent observation shows that the hard parts, of which the mouth is formed, are fully equal to the task.
Teredo navalis.
Teredo navalis.
Perhaps we may say, with propriety, that this is the only species of the shell-bearing tribes that is decidedly injurious to mankind. The animal of the Teredo is a long worm-shaped creature, dwelling in a tube of a shelly substance, which it forms for itself in its progress through the wood; the small pointed shell-like pieces, to the right in the engraving, form the jaws of the animal. With the assistance of these it cuts its way into the timber, and, at the same time, lines the excavation it is making with a shelly substance, which is gradually formed into a tube, the animal occupying that part which is most deeplysunken in the timber; in directing its course it generally excavates in the direction of the grain of the wood, but in some instances it crosses this grain.
In Holland a great part of the country is below the level of high water, and, to prevent the irruption of the sea, immense dykes have been formed along the coast; these are framed, on the sea-side, of large masses of sand, while to the landward they are strengthened by means of strong piles driven into the ground and wattled together. These piles were once discovered pierced in all directions by this destructive worm, to such an extent as to endanger their safety, and had it not been for a timely discovery of the mischief, immense tracts of country would have been laid under water, and irretrievably lost.