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

It is true that the continual dashing of the billows will, in time, destroy the hardest rocks, and that even a granite cliff can not withstand the action of water. But the process of destruction—if we may use such a word—is greatly accelerated by the multitudinous holes bored by the Pholas and other burrowers, and the rock is rapidly undermined by the joint action of the mollusks and the waves. The upper portion of the cliff is thus left without support, and when a few heavy rains have loosened the earth, down comes a mixed mass of rock, soil, and herbage, and a fresh face is thus made to the cliff. The loose earth is soon washed away by the waves, the submerged portion of the fallen rock is eagerly seized upon by the burrowers, and in process of time the whole mass crumbles away, is broken up by the storms, and the fragments are gradually rubbed to atoms by the action of the waves. Thus it is that the cliffs recede on one side, while the soil advances on another, and so the whole face of the world is gradually changed.

Perhaps the DATE SHELLS are even more powerful as burrowers than the mollusks which have just been mentioned. One species, the Fork-tailed Date Shell (Lithodomus caudigera), is able to bore into substances which the pholas can not penetrate. It is truly a wonderful little shell. Some of the hardest stones and stoutest shells are found pierced by hundreds of these curious beings, which seem to have one prevailing instinct, namely, to bore their way through every thing. Onward, ever onward, seems to be the law of their existence, and most thoroughly do they carry it out. They care little for obstacles, and if one of their own kind happens to cross their path, they quietly proceed with their work, and drive their tunnel completely through the body of their companion.

The precise method employed in excavation is at present unknown, for the shape of the shell, and the exactitude with which it fits the burrow, prove that the mollusk does not form its tunnel by means of the protuberances on the surface of the shell, and no other method of boring has at present been discovered.

There is another notable burrower among the bivalve marine shells, which is remarkable for the depth to which it bores, and the hard nature of the substances through which it makes its way. This is the shell called Saxicava rugosa; one of the most variable of the mollusks, so variable indeed that no less than fifteen different names have been given to it, each being supposed to be a separate species. Not only species, but even genera have been formed from the varieties of this curious shell. It is a flattish bivalve, of no very great size, symmetrical in shape when young, but oblong when old.

This creature burrows as rapidly as the species which has just been described, and the process by which the feat is accomplished is quite as enigmatical. Several conchologists have expressed an opinion that the animal must secrete some liquid solvent, which softens the rock, and permits the shell to pass. But, although it is possible for the boring snail to excavate by such means, the Saxicava can hardly do so. For the boring snail is a terrestrial species, and would be able to employ a solvent without interruption; but as the Saxicava works below the surface of the sea, any solvent which it could employ would be either washed away, or so diluted by the water that it would have no effect upon the stone.

Still, that the creature must employ some means not yet known to naturalists is evident from the shape of the hole, and the comparative hardness of the shell and the substances in which it is imbedded. The shell is of ordinary hardness, while the rock in which it is found is often of adamantine density. Sometimes it bores into corals, frequently into limestone, and often into shells, which it penetrates as deeply as the date shell. On every rocky shore which the Saxicava inhabits its burrows may be found, no matter what may be the hardness or composition of the stone. The clay iron-stone which is found about Harwich, and is popularly called cement-stone, is filled with the burrows of the Saxicava. Its tunnels are found in the Kentish rag, while even the well-known Portland stone, of which the Plymouth breakwater is constructed, is often honey-combed by the multitudes of these bivalves that inhabit it. Some of the enormous stones which were employed in building the breakwater are now much wasted by the holes made in them by the Saxicava.

As is the case with the burrows of the date shell, those of the Saxicava do not run parallel with each other, but are driven into the stone at any angle. In consequence of this custom, it is not of unfrequent occurrence that one of the creatures hits upon the burrow of another, and if it does so, it will not, and in fact can not, alter the direction of its tunnel. Neither is it able to wait until the other shell has burrowed farther, but eats its way silently and unrelentingly along, cutting through the shell and body of its luckless companion, and thus bringing on it a violent death, which its rocky home seems especially intended to avert. The hole is on the average about five or six inches in depth, and the animal does not lie free in its burrow, but attaches itself to the side by means of a byssus or cable, like that of the mussel, save that it is smaller, because the strain upon it is not so great as when the shell is anchored in the open sea. This shell has a very wide range of locality, and is sometimes found at a very great depth, specimens having been procured at a depth of nearly 900 feet. It attains its largest dimensions in the colder seas.

Another member of this family (Xylophaga dorsalis) burrows, as its name implies, into wood and not into stone. It is a small species, and of a very globular form, and never burrows to any great depth, an inch being the ordinary length of its tunnel. The shells of this creature are often found in floating wood, or in the sea-covered portions of wooden piles, and it is a notable fact that the burrows are always made across the grain of the timber in which it lives.

Those who are fond of wandering on the sea-shore, will often have experienced tangible proofs of the existence of another burrowing mollusk, the RAZOR SHELL (Solen ensis).

In some parts of our coast it is impossible to walk on the mixed rock and sand, when the tide has receded, without noticing innumerable jets of water, which start from the ground without any perceptible cause, leap for a foot or so in the air, and then disappear. On watching one of these miniature fountains, and looking at the exact spot whence it proceeds, two little round holes are generally seen in the sand, so close to each other as to resemble a key-hole, and large enough to receive an ordinary goose-quill. If the finger be placed on the spot, or even if the foot descends heavily on the ground, the curious object vanishes far out of the reach of a probing finger. The jets are thrown up by the Solen, and the two little holes are the open extremities of the siphon, that wonderful instrument through which the creature obtains its nourishment.

If a Razor Shell should be required for any ordinary purpose, such as baiting fishing-hooks, it can easily be procured by pushing into the hole an iron rod turned up at the end, and twisting the Solen out of its burrow. If, however, a perfect specimen of the animal or shell be required for scientific purposes, it can be obtained by the simple process of dropping a spoonful of salt down the hole. The Solen has a strange hatred of salt, and as soon as the obnoxious substance is felt, up comes the Razor Shell in a hurry, thrusting itself out of the hole, and enabling the operator to seize it before it can again withdraw to its shelter.

The curious group of mollusks called Gastrochænidæ deserve a passing notice. All the species of this family are burrowers, and some of them are capable of making their way through substances of considerable hardness. A common British species, the FLASK SHELL (Gastrochæna modiolina), is notable for its habit of burrowing through various shells, those of the oyster being often perforated and fixed to the creature by some natural cement. In such cases, the animal constructs a flask-shaped case from any loose materials that are within reach, and has by this means earned its specific name of modiolina.

In two remarkable genera the shell is very small, and is cemented to the base of a long shelly tube, through which the siphon runs. The Clavagella is remarkable for the successive frills that decorate the tube; from three to six of these curious appendages being seen in various specimens. These frills are formed by the orifice of the siphon when the tube is elongated. It is a remarkable fact, that the left valve of the shell is always cemented to the side of the burrow, so that the animal possesses no locomotive power, and in all cases the shell is very small, and sometimes scarcely visible.

The WATERING-POT SHELL (Aspergillum) is well known to conchologists. In this creature the shell is exceedingly small, and so deeply sunk into the tube that only the umbo of each valve is visible. The base of the tube is expanded into a rounded and perforated disk, which bears a remarkable resemblance to the “rose” of a watering-pot, and its opposite extremity is mostly decorated with frills, from one to eight in number.

The reader will remember that the wood-boring pholas always makes its burrow across the grain of the timber which it is commissioned to destroy. The SHIP-WORM (Teredo navalis), on the contrary, always burrows with the grain, and never makes a transverse tunnel, unless turned from its course by some obstacle, such as a nail, or the burrow of another Teredo.

At first sight, few would perceive that the Ship-worm belongs to the same class as the oyster and the snail, for it is long, slender, and worm-like in shape, from six to eight lines in diameter, and nearly a foot in length. One end is rather larger than the shaft, if we may use the term, and is furnished with a pair of curved and very narrow shell-valves, while the other is divided into a forked apparatus containing the siphon. The color is grayish-white.

Such is the aspect of the Ship-worm when adult, but in its early stages of existence it possesses a totally different form. When it first issues from the sheltering mantle of its parent, it is a little, round, lively object, covered with cilia, like a very minute hedgehog, and, by the continual movement of these appendages, passing rapidly through the water. It does not, however, retain this form for more than six-and-thirty hours, but undergoes a farther process of development, and is then furnished with a distinct apparatus for swimming and crawling. It also possesses rudimentary eyes, and in that portion of the body which may be considered the head there are organs of hearing resembling those of certain mollusks. When it has passed its full time in this stage of development, it fixes upon some favorable locality, and then undergoes its last change, which transforms it into the worm-like mollusk with which naturalists are so familiar.