Myriapoda.—In the Centipedes, the first evidence of a well-formed extremity makes its appearance in the form of numerous articulated legs terminated by simple points. These legs, rudimentary in the lowest species (Iulidæ), in harmony with their retiring habits and vegetable diet, are divided into distinct joints, and provided with proper muscles, in the strong and carnivorous varieties (Scolopendridæ). In these latter animals, the ventral nervous ganglia are also increased in size, in a ratio proportioned to the higher and more complex structure of the appendages to the trunk.
Insects.—In insects, creatures distinguished by so many and such admirable instincts, and endowed with such active powers of locomotion, the appendages to the body, both legs and wings, are perfectly and beautifully formed. The former, six in number, are attached to the three first thoracic segments of the body, composed of distinct articulations, connected by ligaments and muscles, and furnished with a series of minute breathing tubes, or tracheal ramifications, extending throughout their entire substance. The latter—the wings—vary much in their number, form, and arrangement; and it is upon characters furnished by them that the nearest approximation to a perfect classification of insects—that of Burmeister—is based. Thus, the legs are typical of the class, the wings characterise the orders.
Arachnida.—In the spider tribe (Arachnida) the legs, in their general structure, resemble those of insects; but they are eight in number, and, in addition to them, these creatures are provided with remarkable claw-like appendages to their jaws—mere organs of feeling in the mites, but strong and powerful instruments of prehension in the scorpion tribe.
Crustacea.—The crab and lobster tribe (Crustaceans) are likewise accurately characterised by the appendages to the trunk, the number and disposition of these parts having been employed by Milne Edwards for the purpose of dividing these creatures into a number of orders.
Mollusca.—Among the mollusks, the classes are nearly all named from the peculiarities in form or the position of the feet. We have the barnacles (Cirrhopoda), animals with six pair of articulated flexible arms, composed of a series of small pieces studded with minute hairs, and which form, when they are extended, a species of net by means of which the animal catches its prey. The Branchiopoda—creatures such as the Terebratula—with two long fleshy arms placed near the mouth, which are hollow, muscular, and disposed in spiral folds. We have the Tunicata, such as Ascidians and the Conchifera, as the oyster and mussel, beings which are destitute of members, and all but incapable of locomotion. The Gasteropoda, of which the slug and snail afford familiar examples, and that are furnished with a locomotive apparatus peculiar to the whole class, and termed the foot or ventral disc. The Pteropoda, nearly allied to the Gasteropoda in their internal organisation, but differing from them in the possession of two broad fleshy expansions, resembling fins, which are attached to the side of the neck, and are well adapted for swimming. And, lastly, we have the Cephalopoda, such as the nautilus and cuttle-fish, creatures with numerous strong muscular arms attached around the head, and in some species provided with sucking discs, by means of which they are enabled successfully to attack and capture the strongest and most active prey. In all these instances we clearly see that the appendages characterise the class, and that, from an acquaintance with their number and arrangement, a naturalist could readily come to a conclusion as to the affinities, structure, and general habits of any animal in the series.
Fishes.—The fishes are divided into two great natural orders—those furnished with a bony skeleton and those in which the framework of the body is cartilaginous. The former division are distinguished by the possession of a swimming bladder, and by the nearly equal developement of the parts corresponding to the extremities—the pectoral and ventral fins. The pectoral fins, situated immediately behind the head, are present in all the osseous fishes, traces of them being found even where they might be supposed to be entirely wanting, as in the eel; they do not, however, attain any very considerable size, rarely equalling, and never surpassing, the magnitude of the ventral fins. In the cartilaginous fishes, on the contrary, not only in those which ordinarily remain quiet at the bottom of the water, as the skate, the sole, the flounder, and other flat fishes, but even in the active and voracious sharks we find no swimming-bladder. In these cases, its place is supplied and its duty performed by the enormously developed anterior member or pectoral fin, which greatly surpasses the ventral fin in size; and it is by means of the active movements of this large hand that these creatures raise themselves from the bottom of the water to the surface.
Reptiles.—In the class of reptiles the characters afforded by the form of the extremities are not less important. Of the four orders into which this class is divided, one—that of the serpents (Ophidia)—is altogether destitute of members, with the exception of the rudimentary traces of extremities met with in some few species (Anguis, Bimanes, and Seps). In another order, that of amphibious animals (Batrachia), as the frogs, toads, &c., the extremities are still typical of certain peculiarities of organisation. Some of these creatures may be said to remain in an imperfectly developed or embryonic condition during their whole existence. Instead of breathing in adult life by means of lungs, as do the more perfectly organised of the batrachia, they retain throughout existence that peculiar structure of the respiratory apparatus—the branchial tufts—which in the latter are present but in the embryo condition. In those of the amphibious animals which breathe by means of permanent branchia (Perenni-branchiata), such as the Lepidosiren and the Proteus, the extremities are either mere filamentary appendages, or at most but very imperfectly-formed organs, and always typical of their comparatively inferior organisation; while, in the higher species, that is, those provided with lungs, and losing their branchiæ in the adult state (Caduci-branchiata), as the frog, the salamander, &c., the extremities are well formed and perfectly developed. The third order of reptiles—that of the lizard tribe (Sauria)—is made up of members very dissimilar both in their appearance and general habits. In the aquatic crocodiles the toes are connected by means of a membrane; like ducks and other aquatic animals, these creatures are web-footed. The alligator, so nearly allied to the crocodile in structure and habits, with the exception that it is more terrestrial in its nature, is furnished with a foot the toes of which are only webbed in half their length, the other half being free and unconnected by membrane, their amphibious mode of life being indicated by their foot. If we compare the foot of these creatures with that of the true lizards, the difference of structure will be found to be considerable. The foot of the true lizard is provided with five toes to both the fore and hind members, while attached to the hinder members of crocodiles and alligators we find but four toes. The toes of the lizard are not connected by a membrane, but are separate, and armed with nails, for these creatures live on land, and have need of well-constructed locomotive organs to enable them to catch their active prey. Among lizards, that remarkable animal the chameleon, that is provided with a particular structure of the skin, whereby its change of colour is effected, is distinguished by the possession of a member, the structure of which is not less peculiar. The toes are arranged in two packets, which are capable of being brought into opposition—a structure admirably adapted to the mode of life of the animal—the climbing of trees and shrubs in pursuit of insects. The ichthyosaurus and plesiosaurus, creatures of the lizard tribe which have now become extinct, are both characterised by the form of their extremities. The former has four members like paddles, each composed of nearly a hundred small bones of an irregular polygonal shape and arranged in a pavement-like form. The latter has also four paddle-shaped extremities, but the number of bones of which each is composed is much less than in the ichthyosaurus—not above fifty, the pieces being at the same time of a different shape, and not arranged in a tessellated form.
Birds.—Among birds the foot is a most important organ, serving at once for support, prehension, and locomotion. Its modifications are accordingly very numerous, and ornithologists have employed the characters presented by it as a basis of classification. Scopoli, in his Introduction to Natural History (1777), proposed a systematic distribution of birds into two divisions founded upon the form of the scales covering the foot; thus, some are furnished with scales, small and polygonal (Retepedes), while others (Scutipedes) have the legs covered in front with unequal semicircular plates. So, again, in the division of birds proposed by Nitzsch, viz. into the aerial, terrestrial, and aquatic, the foot will at once serve to indicate the different orders. We will proceed to examine the special characters afforded by the foot of birds in each of the groups or orders, adopting the classification of Kirby, one of the most recent.
In the first order, the birds of prey (Raptores), which includes the eagle, the vulture, the owl, &c., for the purpose of striking their quarry, and of securing it by a firm grasp, a strong muscular foot is necessary, and we consequently find that the foot is provided with four short muscular toes, three placed before, and one behind, all armed with long, sharp, hooked talons, those of the thumb and innermost toe being the longest, as the great duty of grasping is mainly performed by them. The claws are also capable of being drawn back, they are retractile like those of the cat; and the scales upon the upper surface of the foot, as well as the skin covering it below, are strong and tough for the purpose of protecting the member from the attacks of the living prey upon which these creatures feed. The foot is also broadly marked, the kind of marking and the size of the scales being especially characteristic of the different genera and families. In the vultures, which from their feeding on carrion do not require such strength of foot, we find a notable deviation from this general character, the toes becoming longer and slighter and the claws much weaker.
In the second order of birds, the perchers (Passeres of Linnæus), which comprehends sparrows, thrushes, larks, &c., the foot is constructed for another and a very different purpose. Its chief office is to support the bird in a horizontal position upon the boughs and branches of trees and shrubs; hence the foot is long, the toes slender, very flexible, and armed with long, straight, weak claws, which are not at all retractile. All the birds of this order possess four toes, one of which, the hind toe (hallux), is placed straight out behind, the others being in front, and those situated externally are connected to the middle ones at their bases. The scales covering the foot are regularly disposed, and as they are not needed for the purpose of defence, they are thin and tender, resembling ordinary scarf skin. Even in the shrikes,—the birds of prey of this order, who feed upon living creatures and use the foot for securing them, the claws are not sharp, nor is the foot formed in such a manner as to ensure a very secure grasp, and we accordingly find that they merely press this organ upon their prey for the purpose of holding it. In a few other families of this order, as the todies, the kingfishers, &c., the foot is called upon to assist in feeding, and we consequently find certain modifications of structure indicative of this office. In order to strengthen the foot and to secure a firmer grasp, the external and middle toes are united almost to the apex, and the strength of the hind toe is considerably increased.