with which they strike the water vertically, and give an ascending impulse to their bodies. Among the molluscs, the motions of the snail are familiar to every one. They are effected by what is called its foot, or a mass of muscular fibre, situated on the strong membrane which contains the entrails, and also attached to the shell. It glides along the surface, partly by forming a vacuum by means of this organ, and partly by a viscid mucillage secreted by the part. It is thus, also, that some bivalve malluscs, as the common cockle, mussels, razor shell-fish, and others, progress-the animal protruding its foot beyond the shell, and crawling along upon it; and it is furnished also with the same kind of adhesive mucilage, for the purpose not only of steadying its steps during motion, but also, as drawn out into threads under the name of byssus, of preventing it, when at rest, from being washed away, by tides and currents, from the rocks to which it attaches itself.

Advancing in this great class, we find some animals, as the cuttle, moving by a kind of arms or tentacula attached to their head, and employed as oars, or as feet, when moving along the bottom of the sea. On account of the singular place of attachment of the feet, the animals of this, the highest order of molluscs are called Cephalopods (Gr. kephale, head, and pous, foot). With the exception of the pearly nautilus (Nautilus pompilius), which has many tentacular organs attached to the head, all other cephalopods have eight arms; to which, in some kinds, as, e. g., the calamary and sepia, two long and slender tentacula are added, which can be retracted into sheaths. Both the eight ordinary arms and the two tentacles are provided with suckers, by which the animal can attach itself at pleasure. The paper nautilus (Argonauta), has but eight feet, and one pair of these expand at their extremities into broad and thin membranes; the fabled use of which has afforded a beautiful subject for poetic imagery in all ages; but similar appendages occur in Octopus violaceus, and in Octopus velifer, in which both the first and second pairs of feet support broad and thin membranes at their extremities. Now, neither of these species inhabit a shell in which the expanded membranes could be used to waft the animal along the surface of the ocean, as has been said or sung of the Argonaut, from Aristotle to Cuvier, and from Callimachus to Byron.

The comparative anatomist, who has devoted most attention to the structure and economy of the class of Cephalopods, has concluded that "the physiologist, in contemplating the structure of the velated arms, is compelled to deny them the power of being maintained erect and expanded to meet the breeze. What their real function may be is still to be determined; but the removal of the erroneous impressions entertained on the subject is the first step towards the attainment of truth."* Since the article from which the above passage is quoted was published, it has been shown that the membranous arms of the argonaut are the organs for secreting and repairing the shell. This function of the supposed sails of the paper nautilus has been determined by the experiments, instituted, at the suggestion of Professor Owen, by Madame Power, at Messina in Sicily. One of the "sails" was cut off in several living specimens; the right sail being removed in some, the left in others; and the creatures were then kept in a sub-marine cage, and supplied with food. Some of them survived the operation four months, when it was found that the shell had grown only on that side on which the membranous arm had been preserved. By these and other observations it has been finally determined, not only that the argonaut is the veritable constructor of the beautiful and delicate shell which it inhabits; but that its expanded membranous arms never act as sails * See Professor Owen's Article, "Cephalopoda," Cyclopædia of Anat. and Physiol., vol. i. (1836), p. 527.

to catch the wind. While the Glaucus, a beautiful little mollusc, of the Indian Seas and Mediterranean, painted in blue and silver, swims

with great swiftness by its conical and oar-like appendages.

Passing now to the Annelida, we find the earthworm progressing by means of setæ, or bristles, attached to the skin, which the animal fixes on the ground, while, by the elongation of the rings which encircle the body, it moves onwards. Then the head is applied and fixed to the ground, and the body, by the contraction of its rings, drawn towards it. In the Nereis we

find numerous tentacula as organs of locomo

tion, by which, and by undulating inflexions of

GLAUCUS FOSTERI.

the body, the animal swims with great rapidity: while the leech, independently of

THE LEECH (HIRUDO OFFICINALIS.)

its power of swimming by ordinary vermiform motion, is furnished with an apparatus for suction at either extremity of its body. By fixing, alternately, one or

the other, and drawing its body towards it the animal advances at pleasure.

The motion of insects is much more perfect than that of any of the preceding

classes, while a calcareous or horny covering gives attachment to muscles of great power, and enables individuals to move with immense force

and velocity. All spiders dart upon their prey with great rapidity, while some species possess the power of conveying themselves to considerable distances by means of threads, which, propelled from their bodies, they cling to, and are wafted upon them by the winds. The crabs move with great rapidity on the ground; but, from the construction of their joints, they can only progress sideways. The lobsters and cray-fish, again, are only adapted for swimming; but the muscles of both are highly organised and powerful. All winged insects have six legs; and many moreover have, either in the course of their legs, or at their extremities, numerous suckers, by which they form a vacuum every time their legs come in contact with any surface. It is in this way that flies crawl upon a perpendicular surface, or on a smooth mirror, or walk along the ceiling of a room. The structure

of these suckers is very beautiful, and is best Sen in the common blue-bottle fly (Musca vomi

3.

1. Suckers of Blue-bottle Fly. Great Water Beetle. Yellow Saw Fly.

2.

3.

taria), the great water beetle (Ditiscus marginalis), and the yellow saw-fly (Cimex Lutta). But the most remarkable organs of locomotion in insects are their wings. Of these, however, it is sufficient here to say, that they are moved by muscles of immense powers, and that the velocity with which they are moved is at least as remarkable as the force.

Fishes. We next come to Fishes, most of whom effect locomotion by their fins, and of these they employ chiefly the pectoral and ventral pairs, which are strictly analogous to the upper and lower extremities of the superior tribes of animals. Some fishes effect their progression by the motion not of the fins but of the spine; as the lamprey, which has neither pectoral nor ventral fins, and which seems to move in its natural element, the mud, entirely by the lateral flexion of its spine, which it first draws into an S-like curve, and then shoots forward the anterior portion. The same is the case, also, with the eel, when it creeps on land. Others again, as most flat fishes, which, like the lamprey, have neither pectoral nor ventral fins, use their tails principally in making progress in the water. This operation is extremely simple. Everybody knows that the ordinary way of propelling forwards a boat is by rowing; that is to say, by means of one or more pairs of oars passed over its sides, the action of which is exactly similar to the pectoral fins of fishes. But it is likewise well known that a boat may be, with equal certainty, urged forwards by what is called sculling; that is to say, by means of one oar passed over its stern, and continually moved in the water from side to side. Now, it is precisely upon this latter principle that the tail of fishes, moving

MOTION OF FISHES.

from side to side, operates in propelling them forward. It is evident that the oar on the one hand, and the tail on the other, in this alternate lateral motion, is continually displacing a quantity of water great in proportion to the length of the instrument employed, and consequently to the sweep which it makes in its oscillation; and it is by the resistance which the water makes to this displacement, by the oar or tail, in coming from its extreme sweep to the axis or mesial plane of the boat or fish, that either is urged onwards. "Let us suppose," says Dr. Roget, "that the tail is slightly inclined to the right, as shown in the annexed figure. If in this situation the muscles of the left side, tending to bring the tail in a right line with the body, are suddenly thrown into action, the resistance of the water, by re-acting against the broad surface of the tail in the direction P R perpendicular to the surface, will cause the muscular action to give the whole body an impulse in that direction, and the centre of gravity, C, will move onwards in the direction C B, parallel to PR. This impulse is not destroyed by the further flexion of the tail towards the left side, because the principal force, executed by the muscles, has already been expended in the motion from R to M, in bringing it to a straight line with the body; and the force which carries it on to L is much weaker, and therefore

occasions a more feeble re-action. When the tail has arrived at the position L, indicated by the dotted outline, a similar action of the muscles on the right side will create a resistance, and an impulse in the direction of K L, and a motion of the whole body in the same direction, CA. These impulses being repeated in quick succession, the fish moves forwards in the diagonal, CD, intermediate between the directions of the two forces."

It should be added to this description, however, that fishes in general have the power of "feathering" their tails-that is to say, of so puckering up the lobes in their outward motion, as to make them displace as little water as possible-since the effect of the resistance of the water in this direction must obviously be that of retarding their course; while, on the other hand, they expand these lobes on their return to the mesial line, in order that they may displace as much water as possible; since it is upon the re-action of the water in this direction that they rely for their advancement.

The bodies of fishes are of very nearly the same specific gravity as the water in which they live, owing to the great quantity of fat which most of them contain; so that very little effort is required to keep them at any given height, and their descent or ascent in the water is comparatively easy; the latter being further promoted by the faculty they possess of filling their air-bladder at pleasure with air. When they attach themselves to rocks, it is by means of suckers, as in other tribes; and when they leap from the surface of the water, it is by a sudden and forcible extension of their bodies after a strong flexion, the elasticity of the water thus giving them the force of a projectile. Some fishes also, as the flying kinds, are capable of using their long fins in the air almost in the manner of the wings of birds-a hundred yards being no unusual flight.

Reptiles. In serpents we find the spine as an organ of locomotion; and these, unlike all other vertebrata, have only abdominal and caudal vertebræ, the motions of which are exceedingly free upon each other. Serpents differ from fishes, in the circumstance of their spine supporting their true ribs, as well as in that of the bodies of their vertebræ being attached to each other, not by means of an interposed fluid, contained in a shut cavity formed by the juxtaposition of the bones, but by means of a rounded head on the posterior part of the body of each, which is received

into a corresponding socket on the anterior part of the one behind it. The spinous processes, also, of the vertebræ of serpents being in general considerably shorter than those of most fishes, the motions of their spinal column are not only lateral, but in a great measure upwards and downwards also; although some painters and statuaries appear to have a little overdone this matter, and to have represented flexures of the bodies of serpents where no countenance can be given to them by anatomy. There are limits, in this respect, beyond which we cannot allow even the sublime hand of the sculptor of the Laocoon to pass without reproach. But although the motions of serpents are thus very similar to those of "the wandering eel" in its peregrinations on the grass, the former has an assistance which the latter has not, and one which appears to constitute a link in the transition of the spine, as an organ of locomotion, very similar to that constituted by the bristles of the aphrodita, or sea-mouse, in the same transition in the invertebrate. This assistance is the ribs, which in serpents are, in fact, organs, not so much of respiration-as is the case in most lizards, in birds, and in mammiferous

VERTEBRA OF SERPENT.

92

66

RIBS OF SERPENTS ORGANS OF LOCOMOTION.

animals-as of progression; and the raising or drawing forward of these ribs corresponds more to advancing a leg than to any other motion of animals furnished with proper limbs. Not only, then, is the spinal column of serpents a means of helping them forwards, but the ribs being at the same time drawn towards their head, the transverse plates on the lower surface of the body, with which they are connected, move forward, as it were, like so many feet, when the spine and the rest of the body are drawn forwards upon them. It is thus that the serpent fulfils the curse pronounced upon the first tempter, Upon thy belly shalt thou go, all the days of thy life." It is the only "beast of the field"-in other words, the only terrestrial vertebrated animalwhich does not employ legs as an organ of progression; but the transition, in this respect, from the serpent to those vertebrated animals in which these organs are both structurally and functionally most developed, is extremely insidious, and furnish another striking illustration of the axiom, that nature never advances by sudden leaps in her productions, and that she knows no chasms in the chain of creation. She has given even to the serpent a kind of rudimentary legs, or, at least, arms, manifesting themselves in a kind of claws, situated in most species under the common covering of the body, but in some few projecting a little beyond it; and in some kinds of lizards-the snake-lizard, for example-the improvement in this respect is hardly appreciable. The legs of this animal are scarcely less rudimentary than those of the serpents; and, like the caterpillar among the invertebrate tribes, it probably advances, at least equally, by means of its spine, as by means of its legs. A further step is gained in the case of the land salamander, which, like the centipede, uses its legs-which are considerably more developed than in the snake-lizard-more than its spine as an organ of locomotion; but its legs are still almost as often tilted up into the air as resting on the ground, in the process of walking, and it is not until we come to some of the higher tribes of lizards-as to those of insects in general-that we find the legs exclusively and unequivocally employed as instruments of progression.

RIBS OF SERPENT.

Some lizards, also, move up perpendicular surfaces by a species of suction; the soles of their feet, as in the gecko, being provided with a series of soft plates, which, being drawn up at pleasure, produce the requisite vacuum. Other reptiles-as the tortoise-make progress on land by crawling, and the frog by crawling and leaping; others as the flying-lizard-use their ribs, not, like serpents, as legs, but as wings. In the water most reptiles use their legs as fishes do their fins; and some of them, as turtles, keep themselves afloat by a collection of air beneath their dorsal shield.

Birds. When on land, the progression of birds is effected by either walking or hopping on their posterior extremities only, birds being the only proper bipeds among