directly engaged in the living functions of the tree, is yet essential for extending the branches and leaves to the influence of the atmosphere, and by its elasticity under the pressure of the wind, giving what is equivalent to exercise for the motion of the sap. A tree opposed to winds and to a severe climate is dense in its grain, and the wood is preferred by the workman to that which is the growth of a milder climate.

We cannot miss seeing the analogy of the woody fibre to the bones of animals. Bones are firm, to sustain the animal's weight, and to give it form. They are jointed, and move under the action of muscles; and this exercise promotes the activity of the living parts, and is necessary to health. But let us first observe the structure of some of the lower animals. It will be agreeable to find the hard material, though always appropriate and perfect, becoming more and more mechanical and complex in its construction, from the lithophytes, testacea, crustacea, reptiles, fishes, mammalia, up to Man.

The first material to be taken notice of, which bestows this necessary firmness on the animal textures, is the cellular substance. This consists of delicate membranes, which form cells; these cells communicate with each other, and the tissue thus composed enters everywhere into the structure of the animal frame. It constitutes the principal part of the medusa, which floats like a bubble on the water; and it is found in every texture of the human body. It forms the most delicate coats of the eye; and gives toughness and firmness to the skin. It is twisted into ligaments, and knits the largest bones: it is the medium between bone, muscle, and blood-vessel: it produces a certain firmness, and union of the various component parts of the body, while it admits of their easy motion. Without it, we should be rigid, notwithstanding the proper organs for motion; and the cavities could not be distended or contracted, nor could the vessels pulsate.

But the cellular texture is not sufficient on all occasions, either for giving strength or protection: nor does it serve to sustain the weight, unless the animal lives suspended in water, or creeps upon the ground. Shell-fish have their strong covering for a double purpose: to keep them at the bottom of the sea, and to protect them when drifted by the tide against rocks. Those animals of the molluscous division which inhabit the

deep sea, and float singly, or in groups, as the genus scalpa, have a leathern covering only; because they are not liable to the rough movements to which the others are subject, in the advancing and retiring tides. The scalpa, simple as it is in structure —for it presents the appearance of a mere bag with two orifices capable of opening and closing by valves-possesses at once all the functions of digestion, respiration, reproduction, and, more strange than any, locomotion; in its outward form and substance, we may see the provisions for its mode of life, and the place that it holds: from floating or swimming at will, it is one of the "natantes ;" and it is further distinguished by the term “tunicata,” from being furnished with a leathern coat : now it is worthy of admiration, that although unprovided with exterior members, and having only two or three muscular bands attached to its outward covering, it can move from place to place, by merely taking in, and throwing out, the water in which it floats; and the same operation is sufficient to supply it with its food, and carry on the process of respiration.

The hermit crab gives us a demonstration of the necessity for a protecting covering. Its tail or hinder part has no crust or shell upon it, as its body and claws have; therefore this animal requires to seek a suitable dwelling-place for itself—some empty univalve shell, into which it insinuates its tail, and from which its head and arms project: with this power of selecting a house, it removes, when it has outgrown the shell in which it has dwelt; and may then be seen trying the empty shells upon the shore, or contending with others of its own species for the possession of a shell. Surveying these instances, we cannot resist the conviction of the fine adaptation of the sensibilities and instincts of animals to their forms and substances.

With all this, when we look to animals of more complex structure, possessing a distinct system of muscles, we perceive the necessity for some harder and more resisting material being added, if the weight is to rest on points or extremities; or if the muscular activity is to be concentrated. And nature has other means of supplying the fulcrum and lever, besides the bones, or true skeleton, which we have been examining in the first part of the volume. Perhaps we shall find that there is a system of solid parts superior even to what we have been studying in the vertebrata.

The larvæ of proper insects, and the annelides, have no exterior members for walking or flying: but to enable them to creep, they must have points of resistance, or their muscles would be useless. Their skins suffice; and these are hardened by a deposit within them, for that purpose. But if this skin were not further provided, it would be rigid and unyielding, and be no substitute for bone. The hardened integuments are, therefore, divided into rings; to these the muscles are attached; and as the cellular membrane between the rings is pliant, the animals are enabled to creep and turn in every direction.

Without further argument, we perceive how the skin, by having a hard matter deposited within it, is adapted to all the purposes of the skeleton. It is worthy of notice that some animals, still lower in the scale—the tubipores, sertularia, cellularia, &c., exhibit something like a skeleton. They are contained within a strong case, from which they can extend themselves; whilst the corals and madrepores, on the other hand, have a central axis of hard material, the soft animal matter being, in a manner, seated upon it. But these substitutes for the skeleton are, like shell, foreign to the living animal; although in sustaining the softer substance and giving form, they may resemble bone.

The texture of a sponge, its form and elasticity, depends upon a membranous and horny substance, to which both silicious and calcareous spiculæ are added. Of shell, the hardening material is carbonate of lime, united to a membranous or cartilaginous animal matter. Paley describes the slime of a snail hardening into shell by the influence of the atmosphere: but that is a very imperfect, and indeed erroneous view. The shell of the oyster, and even the pearl, consists of concentric layers of membrane and carbonate of lime; and it is their laminated arrangement which causes the beautiful iridescence in the polished surface of those shells.* In the rough outer surface of an oyster shell, we shall see the marks of the successive layers: that which now forms the centre and utmost convexity of the shell was at an earlier age sufficient to cover the whole animal; but as the oyster grows, it throws out from its surface a new secretion, composed of animal matter and carbonate of lime,

* See the discoveries of Sir David Brewster on this subject: Phil. Trans. 1814, p. 397.

which is attached to the shell already formed, and projects farther at its edges. Thus the animal is not only protected by this covering, but as it increases in size, the shell is made thicker and stronger by successive layers.

The reader will not be unwilling that we should stop here to show that, rudely composed as this covering of the oyster seems to be, it not only answers the purpose of protecting the animal, but is shaped with as curious a destination to the vital functions of respiration and obtaining food, as anything we can survey in the higher animals. We cannot walk the streets without noticing that, in the fish-shops, the oysters are laid with their flat sides uppermost. They would die were it otherwise. The animal breathes and feeds by opening its shell, and thereby receiving a new portion of water into the concavity of its under shell; and if it did not thus open its lid, the water could neither be propelled through its branchiæ or respiratory apparatus, nor sifted for its food. It is in this manner that they lie in their native beds; were they on their flat surface, no food could be gathered, as it in their cup; were, and if exposed by the retreating tide, the opening of the shell would allow the water to escape, and leave them dry, thus depriving them of respiration as well as food. We perceive, then, that the form of the oyster-shell, rude as it seems, is not a thing of chance. Since the shell is a cast of the body of the animal, the peculiar shape must have been given to the soft parts, in anticipation of that of the shell; an instance of prospective adaptation.

That the general conformation of the shell should have relation to what we may term its function, will be less surprising, when we find a minute mechanical intention in each layer of that shell. We should be inclined to say that the earthy matter of the shell crystallises, were it not that the striated or fibrous appearance differs in the direction of the fibres in each successive stratum-each layer having the striæ composing it parallel to one another, but directed obliquely to those of the layer previously formed, and the whole exhibiting a strong texture arranged upon well-known mechanical principles.

* In confirmation of these remarks, when the geologist sees the fossil shells in their strata, he can determine whether the oysters were over

whelmed in their native beds; or were rolled and scattered, as shells merely.

Shell is not alive, as true bone is. If the shell of any of the testacea be broken, the surface of the animal secretes a new shell: not, however, by the concretion of slime, but by the regular secretion of a substance combined of earthy and gelatinous matter.* Delicate experiments have been made by steeping shells in diluted nitric acid, by which it is shown that the carbonate of lime is the earthy material of shells; and that, when that earth is dissolved in the acid, a gelatinous substance of the form of the shell remains.

In crustaceous animals, such as the lobster and crab, the shell is formed of the same substances as the testacea, but with an addition of the phosphate to the carbonate of lime. A question arises, how these animals grow? It is found that they cast off their shells, and remain retired until a new and larger shell is secreted. Reaumur has given a very particular account of the process of separation, in the cray-fish. † In the shell of crustacea, we find an approximation to bone, inasmuch as it is articulated, and has certain processes directed inwards, to which the muscles are attached.

In the insect, the resisting material is deposited externally, and is converted to every purpose attained by means of an osseous system. Distinct members are formed, with the power of walking, leaping, flying, holding, spinning, and weaving. The hardened integuments, articulated and performing the office of bones, have like them spines and processes: with this difference, that their aspect is towards the centre, instead of projecting exteriorly. Were we to compare the system of resisting parts in man, and in the insect, we should be forced to acknowledge that the mechanical provisions in the lower animal are superior! The first advantage of the skeleton in the insect being external, and removed from the influence of the circulation, is, that it is capable of having greater hardness and strength imparted to it, according to the necessities of the animal, than can be bestowed upon bone. True bone, being internal, and depend

*We owe our knowledge of the formation of shell to the great French naturalist Reaumur; who, by ingenious experiments, showed the distinction between shell and bone, and that the former was secreted from the surface of the animal.

See a paper by Sir John Dalzell, on the Exuviation of the Crustacea: Transactions of British Association, 1851, p. 120.

It is termed "exoskeleton," as contrasted with the "endoskeleton," or internal skeleton.