Homes Without Hands: Being a Description of the Habitations of Animals, Classed According to Their Principle of Construction - Page 21

Granting that the snail really does form the burrow, we have still to discover the mode of working. Mr. Hancock says that it must do so by means of an acid secretion proceeding from the foot, which corrodes the rock and renders it easy to be washed away. If the snail be removed and placed on litmus paper, the ruddy violet color which at once tinges the paper shows that there is acid of some kind, and if the paper be applied to the spot whence the snail has been taken, the same results follow. It is a remarkable fact that although the snail leaves the usual slimy marks of its progress when crawling in the summer time, no mucus is perceptible on the approach of winter. When the cold months come round, the Boring Snail leaves its food and attaches itself to the rock, remaining in the same spot until summer approaches. During this time, the portion of rock to which it clings is worked away, and the stone around the excavation is impregnated with a greasy matter which soon dries up after the admission of the atmosphere. In a letter to me, dated October 14th, 1863, Mr. Hancock remarks that the rock at Monte Pellegrino in Sicily, which is crystalline and hard as marble, is perforated by the same snail and in the same manner. I may here mention that the stone of the Bois des Roches is that of which the column at Boulogne is built, which has retained its sharpness of outline after exposure to wind and weather for nearly sixty years. It is therefore called marbre Napoléon. Mr. Hancock proceeds to say, “The following are a few of the peculiarities which I have not mentioned in my letter in the Field:

“1st. There is no instance at Bois des Roches of a tunnel being formed on the horizontal surface of a rock, or on the sides facing the south and southeast. They are always on the sides facing the north or northeast.

“2d. The snail forms no epiphragm.”

[The “epiphragm” is the barrier of hardened mucus with which snails mostly close the entrance of their shells. There are generally several in each shell.]

“3d. Though during the summer it leaves behind it the usual slimy mucus track; in the winter, on returning to the rocks, no track is perceptible except the corrosion of the rock by frequent passage. This would seem to point to a system of secreting organs for the acid, separate from that for the mucus.

“4th. Contrary to the usual habits of burrowing mollusks, who generally have a bed of muddy matter between their shells and the walls of their dwelling, the Helix saxicava keeps his tunnel perfectly clean and neat.

“5th. When the liquor alluded to as forming a fatty aureole round the tunnel penetrates into pre-existing clefts in the rock, it provokes the growth of a microscopic lichen, which also grows in the tunnels in places after the liquor has evaporated.

“6th. The tunnels of the Helix saxicava are always irregular, bearing no relation to the size or shape of the excavators, whereas, in other excavating mollusks, the shape of the hole always bears some relation to its occupant, and also the excavations are alike for all animals of the same species.”

There is an opinion that the gastric juice secreted in the stomach may be the means through which the tunneling is conducted, and that instead of being employed as food within the body it is poured out upon the stone, so as to dissolve it, the softened substance being then removed by the foot. The Boring Snails do not congregate together during hibernation, as is the well-known custom of the garden species, but are always solitary. Sometimes two or even three are found in the same burrow, but then they are always at some distance from each other, and form supplementary tunnels of their own. In my own specimen there is a curious example of this peculiarity, where the snail has contrived to bore completely through the barrier that separates it from a neighboring tunnel, and has made a hole as large as the key-hole of an ordinary writing-desk, and nearly of the same shape.

There are many marine-boring mollusks, some of which excavate mud, others stone, and others timber. Of the mud-borers I have little to say, few of them possessing points worthy of notice. Perhaps the most noteworthy of these is the common GAPER SHELL (Mya arenaria), so called because one end of the shell gapes widely, in order to permit the passage of a long and stout tube. In a specimen now before me, the tube is between three and four inches in length, and at the base is large enough to admit the thumb. As, however, it gradually tapers to the extremity, the aperture at the other end is scarcely capable of receiving the little finger. The walls of this tube are very thin and membranous, and it is more or less retractile, carrying within it the siphons through which the mollusk respires and takes nourishment.

The Gaper Shell inhabits sandy and muddy shores, and to an inexperienced eye is quite invisible. The shell itself, together with the actual body of the mollusk, is hidden deeply in the mud, seldom less than three inches, and generally eleven or twelve inches from its surface. In this position it would be unable to respire, were it not for the elongating tube, which projects through the mud into the water, and just permits the extremities of the siphons to show themselves, surrounded by the little radiating tentacles which betray them to the experienced shell-hunter. These tentacles or fringes are never seen in the dried specimens, and can only be partially preserved by plunging the animal into spirits of wine, glycerin, or other antiseptic liquid. The Gaper Shell is esteemed as an article of food by man, beast and bird; for not only do human beings dig it up with tools, cook it, and eat it, but the wolves and the arctic fox scratch it out of the mud and eat it raw, and the various sea-birds peck it out with their beaks, prize the shell open, and devour the contents.

The well-known Limpet is a kind of borer, though the holes which it excavates are of very trifling depth, and are probably made by the mechanical friction of the shell and foot against the rock, without any intention on the part of the animal. Those who have been accustomed to wander along the sea-shore must have noticed that the Limpet shells always sink more or less into the rocks on which they cling, and that in very old specimens which are covered with algæ and barnacles, the shells are often sunk fully half their depth into the solid rock. Grooves, too, of various depths may be seen in the same rock, showing the slow and tedious track which the Limpets have made over its surface, until they finally settled down into some convenient situation.

Our next example of the burrowing mollusks is the well-known PHOLAS, popularly called the PIDDOCK (Pholas dactylus), the shells of which are extremely plentiful upon our coasts, whether empty and thrown upon the beach, or still adhering to the living animal and deeply sunken in the rock. Almost in every part of our shores the Piddock is to be found wherever there is rock, and its dimensions and general appearance vary together with the locality. The chalk cliffs, which bound so many miles of our coast, are thickly studded with the burrows of the Piddock, which takes up its residence as high as the mid-water zone of the coast, and in some places is so plentiful, that the hand can scarcely be laid upon the rock without covering one or two of the holes.

The shell itself is extremely fragile, and of a rather soft texture, and its outer surface is covered with ridges, that sweep in the most graceful curves from the hinge to the edge, and bear some resemblance to the projections upon a file. Yet practical naturalists have proved that, by means of these tiny points and ridges, the Pholas is able to work its way into the rock; for not only can a similar hole be bored by using the shell as a brad-awl is used to pierce wood, but the creature has actually been watched while in the act of insinuating itself into the chalk rock, a feat which was performed by gently turning the shell from right to left, and back again.

The Pholas burrows to a considerable depth, and if a piece of the rock be detached and broken to pieces by the hammer, it will be seen to be completely riddled with the perforations. Chalk rock is mostly the richest in specimens, but even the hard limestone formations are penetrable by the fragile shell of the Pholas. It has been well remarked that the size of the Pholas and the

Pholas in Wood. Lithodomus.

Razor Shell.

Pholas in Rock.

sharpness of its markings vary in inverse ratio to the hardness of the rock in which it burrows. From the softest sea-beds are taken the largest and most perfect shells, while those specimens which are obtained from the hard limestone rocks are comparatively small, and the surfaces are rubbed nearly smooth. The very worst examples, however, are those which are found in gritty rocks, interspersed with pebbles. The shells that have burrowed into such substances, are dwarfed, abraded, and often misshapen, and are valueless except to the physiologist.

We naturally ask ourselves why the Pholas should bury itself in the rocks instead of passing its life in the open sea, like the generality of bivalves. The creature does not feed upon the substances in which it forms its curious tunnel, and to all appearance would obtain as much food without as within the burrow. One obvious answer to this question is, that the creature buries itself in the rock for the sake of safety, its shell being, as has already been remarked, of a soft and fragile texture. This opinion is farther corroborated by the fact that one of the British species, the Paper Pholas (Pholas papyracea), has a peculiarly thin and delicate shell, so as to earn for it the name by which it is popularly known. Yet, although this may be a reason, it is not the reason; for there are many well-known shells which are far more fragile than those of the Pholas, yet which need no such protection, and instead of concealing themselves in any way, roam the ocean freely.

In my own opinion, the burrowing instincts of this and many other marine creatures of similar habits are implanted in them for other than mere individual purposes. Judging by the effects which these animals must have produced upon the line of coast throughout a succession of centuries, I can not but think that they are, at all events partially, intended as instruments which aid in producing those mighty changes that are continually taking place over the whole face of the globe.

We know that although the general proportions of sea and land are maintained, a continual change is being worked in their relative positions. Even within the memory of man, fields now blooming with corn were once covered with the salt water, and buildings that were once a mile from the shore are now in hourly danger of falling into the sea. And I have very little doubt but that the Pholas plays a very important part in these changes. If the reader will examine any of our chalk-bound coasts, and will walk from the foot of the existing cliff to the extreme of low-water mark, he will see that the Pholas has every where left the tokens of its industry. Even at the verge of low water, the spot whereon he stands was once the base of the cliff which has now receded so far from the waves, and continues to recede yearly. And as he looks out to sea, and watches the breakers fling their white foam over the sunken rocks, and notes the dark masses of tangle lifting with the waves, he sees the remains of former cliffs, that have long since crumbled away piecemeal and fallen into the sea.

Page 21 of 120