That part of the tongue of the limpet near the point is, of course, subjected to an amount of wear and tear very different from the other portion. With constant use, teeth, even of flint, must in time wear out. A constant succession of them is, however, most bountifully provided; and there is a continued growth of the tongue at the back part to counterbalance the wear that is going on in front. This reserve part is folded up beneath the viscera of the limpet. When the fore part of the tongue, where alone the teeth have their requisite degree of hardness, is worn down, the part supporting them disappears likewise, and the adjoining part comes into play. The entire number of rows of teeth is about 160, and as there are twelve teeth in each row, the limpet has in all about 1920 teeth.

But this supply, liberal as it is, falls short of that possessed by his air-breathing brethren that live on land. The large black slug (Arion ater) has 160 rows of 110 teeth, making a total of 17,600; and this is exceeded by the large grey slug (Limax maximus), who enjoys 160 rows of 180 teeth, making an aggregate of 28,800 teeth! The hedge snail, with its pretty little banded shell and variety of colouring, has 135 rows of teeth of 100 in each, making 13,500 in all. What wonder that the farmer and the gardener should occasionally bemoan the injury sustained by their crops from assailants so formidably armed!

Now, all those creatures, whether on land or sea, agree in thisthe body is covered by a skin which is soft and moist; this softness of the skin is one of the characteristics of that division of the animal kingdom to which they belong-the mollusca. Again, while the oyster and the mussel have shells consisting of two valves or pieces, others, like the whelk or the snail, have the shell of one piece only; hence the latter are spoken of as univalves, and the former as bivalves. The whelk and the snail agree in another point-they move by a large expanded disk or "foot," placed at the lower surface of the body. From this peculiarity, all animals in which it is manifested are placed in one group, and receive a scientific name (Gasteropoda), which literally means "belly-footed." This large class of molluscs is again divided into orders, according to modifications of the breathing organs.

3. THE BORING COCKLE, AND THE MUSSEL.-Let us return once more to the shore. About a mile from my house there are ledges of hardened clay, which are covered by every tide.

They are pierced with holes, the borer being one of those bivalve shells, to which children give the name of "Lady cockle," and the learned that of Tapes pullastra. A few specimens are carefully dug out, alive and uninjured, are carried home, and placed in a deep plate filled with sea-water. While I am transferring them, the children, by my directions, tear up an old envelope into small fragments. That being done, I take the paper and sprinkle it over the surface of the water. In one moment all the pieces are in motion-one hurrying here, another there, stopping suddenly as though they had forgotten something, then hasting forward with increased speed. They are dancing! hands across, set to partners and turn, down the middle and back! Are the bits of paper bewitched? The children are clamorous with delight, and ask the cause of it all. The explanation is not difficult. Those "shellfish" (to use the common expression) that belong to the same class as the oyster or the cockle, agree in the general structure of the gills or breathing organs. These are placed between the folds of what is called the cloak or mantle, and are strengthened by delicatejointed filaments. These, again, support numerous rows of very minute hair-like bodies, termed cilia, whose ceaseless movements cause currents in the water, and these currents convey to the animal the means of respiration and of nutrition. If any of you can procure a microscope, and can find a common Mussel in a living state, break the shell, cut off a small piece of the branchial fringe, and place it in focus along with a drop or two of sea-water. You will then see a framework of little bars with curious crossjoinings, and thousands of the minute hair-like bodies (cilia) in ceaseless movement. It is a sight which I shall never forget. It excited in my mind the most lively emotion of surprise. when I considered that in the bay were millions of mussels, every one of which contained an apparatus equally perfect, I could not but ponder on the evidence thus afforded of the Power by which they were created, and the Providence by which they were preserved. The humble mollusc, resting on its bed of mud, seemed to speak, in tones not to be misunderstood, of "the works of the Lord, and his wonders in the deep." All bivalves, however, that burrow in sand or mud, are furnished with tubes instead of cilia by one of these the pure stream is received and conveyed to the gills; by the other, the exhausted or impure water is carried away. Sometimes these tubes are separate, sometimes they are

And

enclosed in a common sheath, their length and their power of extension being exactly adapted in all cases to the wants of the animal. If it merely cover itself in the sand, they are short; if it burrow deep, they can be protruded so as to reach the surface. If their free action should be impeded, and the communication between the animal and the surrounding sea-water cut off, death would ensue. We are thus enabled to find a clue to the origin of the mysterious dance. The water was full of currents caused by the little bivalves, each of which had one stream hurrying towards the entrance of its respiratory tube, and one issuing from the other orifice, and consisting of water that had fulfilled its functions and was no longer required.

A boring bivalve of larger size, called the Pholas dactylus, has the mantle prolonged into a thick rounded tube with two orifices, one of which is extremely contractile. These tubes form the only means of communication between the anchorite who inhabits the cell, and the world without. The animal, when very young, commences his excavation, lives in the cavern he has formed, and enlarges it from time to time as his increasing size requires. There his whole life is passed; he is the solitary inmate of the grotto he has constructed. Even were he desirous of leaving his abode, escape would be impossible; for the thick end of the shell, which possesses the perforating power, is placed below. The Pholas might well say, like Sterne's starling, "I cannot get out." Yet he does not want for splendours which more gorgeous dwellings cannot display; for his is lighted up by means of a phosphoric secretion. Superior to our railway engineers, he makes his tunnel with tools that are never out of repair, and illuminates every part of it with a bluish-white light, supplied abundantly, yet free of all expense. On one occasion a friend of mine collected a number of specimens, and carried them home in a botanical box. On opening it at night the interior of the box was one blaze of light!

4. THE SHELL OF MOLLUSCS.-The process of respiration, admirable as it is, and carried on by organs beautifully modified according to the necessities of the several groups, is not the only one which well deserves our consideration. The secretion of the material of which the shell itself is composed offers an interesting subject for inquiry. It is now ascertained that the formation of the shelly matter, and its application for the repair or enlargement

Each

of the shell, is due to the part known as the "mantle." layer of the shell first formed, in fact, a portion of the mantle, either as a membrane only, or as a layer of cells. Hence shells consist in part of animal membrane, and in part of carbonate of lime; the minute cells of which they are built up presenting in different families considerable variety in their size, form, and arrangement. The observations of Lieutenant Maury, of the United States Navy, in his Physical Geography of the Sea, teach us to connect this deposition of shelly matter, whether by the coral builders of warmer latitudes, or the mollusca of our own coasts, with some of those large operations of nature that are ever going forward. The following are his words :

“The dews, the rains, and the rivers, are continually dissolving certain minerals of the earth, and carrying them off to the sea. This is an accumulative process; and if it were not compensated, the sea would finally become, as the Dead Sea is, saturated with salt, and, therefore, unsuitable for the habitation of many fish.

"The sea-shells and marine insects afford the required compensation. They are the conservators of the ocean. As the salts are emptied into the sea, these creatures secrete them again, and pile them up in solid masses, to serve as the basis of islands and continents, to be in the process of ages upheaved into dry land, and then again dissolved by the dews and rains, and washed by the rivers away into the sea."

But they do not merely collect and condense the mineral constituents which the sea-water contains; they are, according to Maury, important agents in causing a continual movement in the particles of the sea-water itself, and causing successive portions of it to rise to the surface. If the sea were in a state of perfect rest, and a single mollusc abstract from it the solid matter for its cell, the specific gravity of that portion is altered. It has become lighter than it was before; it must, therefore, give place to the pressure which the heavier water exerts to push it aside and to occupy its place. "The sea-breeze," says Maury, "plays upon the surface; it converts only fresh water into vapour, and leaves the solid matter behind. The surface-water thus becomes specifically heavier, and sinks. On the other hand, the little marine architect below, as he works upon his coral edifice at the bottom, abstracts from the water there a portion of its solid contents; it, therefore, becomes

specifically lighter, and up it goes, ascending to the top with increased velocity, to take the place of the descending column, which, by the action of the winds, has been sent down loaded with fresh food and materials for the busy little mason in the depths below."

CRABS.

"Oh! what an endlesse work have I in hand,

To count the sea's abundant progeny!

Whose fruitful seede farre passeth those in land."--SPENSER.

1. THE SHORE CRAB.--On the moist sand of the bay, during the summer months, the young of the common Shore Crab (Carcinus manas) are very abundant. They are of various sizes, some of them being so small, that a silver fourpence laid upon "the shell" would cover the entire animal. They are of various colours, too; for while most of those which are full-grown are of a blackish-green above, and of a reddish tinge at the lower part of the body, the "children" sometimes appear arrayed in white, or in a mottled costume of white and black. It is pleasant to note how rapidly they run, and with what quickness they can sink down into the soft sand for concealment. Now and then when we turn up a stone, or lift a mass of sea-weed out of the little pool at its base, a fullgrown crab, disturbed in his retreat, will rush forth, and with up-lifted claws offer battle to all assailants.

Occasionally, we have come upon some poor fellow who had cast his old shell, and whose new one had not yet become hard or crust-like. He generally at such times stays at home, and keeps quiet, as if aware that his usual armour no longer protects him from injury. It is a great pleasure to children at such times, to take the defenceless warrior in their hands, and after satisfying themselves that he is in his undress, and not prepared for visitors, gently to replace him in his chamber-a pleasant nook of a little pool of sea-water, the floor of fine sand, and the drapery of sea-weeds hung with a grace which man would try in vain to emulate.

From this crust or shelly covering, is derived the name of the lass to which crabs and their relatives belong--CRUSTACEA. They