The Vertebrate Exoskeleton

The Exoskeleton never attains, in vertebrated animals, the functional importance which it so frequently possesses among the Invertebrata, and it varies very greatly in the degree of its development.

The integument consists of two layers - a superficial, nonvascular substance, the epidermis, composed of cells, which are constantly growing and multiplying in the deeper, and being thrown off in the superficial, layers; and a deep vascular tissue, the dermis, composed of more or less completely formed connective tissue. An exoskeleton may be developed by the hardening of either the epidermis, or the dermis.

The epidermal exoskeleton results from the conversion into horny matter of the superficial cells of the epidermis. The horny plates thus formed are moulded upon, and follow the configuration of, areas, or processes, of the dermis. When the latter are overlapping folds, the horny epidermic investment is called a scale, squama. When the dermic process is papilliform, and sunk in a pit of the dermis, the conical cap of modified epidermis which coats it is either a hair or a feather. To become a hair, the horny cone simply elongates by continual addition of new cells to its base; but, in a feather, the homy cone, which also elongates by addition to its base, splits up, for a greater or less distance along the middle line of its under surface, and then spreads out into a flat vane, subdivided into barbs, barbules, etc., by a further process of splitting of the primary horny cone.

The epidermis remains soft and delicate in Fishes and Amphibia. In Reptilia it sometimes takes the form of plates, which attain a great size in many Chelonia; sometimes, that of overlapping scales, as in Ophidia and many Lacertilia; but, sometimes, it remains soft, as in some Chelonia and in the Chamaeleons. Epidermic plates in the form of nails appear upon the terminal phalanges of the limbs.

All Aves possess feathers. In addition, the beak is partly or completely ensheathed in horn, as in some Reptilia. Cornified epidermic tubercles or plates are developed on the tarsi and toes, the terminal phalanges of which (and sometimes those of the wing) have nails. Besides these, some birds possess spurs, which are ensheathed in horn, on the legs or wings.

In Mammalia, the horny exoskeleton may take all the forms already mentioned, except that of feathers. In some Cetacea it is almost absent, being reduced to a few hairs, present only in the foetal state. The Pangolin (Manis), on the other hand, is almost completely covered with scales, the Armadillos with plates, and most terrestrial mammals with a thick coat of hair. The greater part of the mass of the horns of Oxen, Sheep, and Antelopes, is due to the epidermic sheath which covers the bony core. Where the horny epidermis becomes very thick, as in the hoof of the Horse, and in the horn of the Rhinoceros, numerous long papillaes of the dermis extend into it. These papillae, however, are comparable to the ridges of the bed of the nail, not to the pajpillos of the hairs.

The dermal exosheleton arises from the hardening of the dermis; in the majority of cases by the deposit of bone-earth, in more or less completely-formed connective tissue, though the resulting hard tissue has by no means always the structure of bone. It may happen that cartilage is developed in the dermis; and, either in its primary state or ossified, gives riso to exoskeletal parts.

No dermal exoskeleton (except that of the fin-rays) is found in the lowest fishes, Amphioxius and the Marsipobranchii. In most Teleostei, the integument is raised up into overlapping folds; and, in these, calcification takes place in lamina, of which the oldest is the most superficial, and lies immediately beneath the epidermis. As a general rule, the calcified tissue of the "scale" thus formed, does not possess the structure of true bone in the Teleostei. But, in other fishes, the dermal calcification may consist of true bone (as in the Sturgeon); or, as in the Sharks and Rays, may take on the structure of teeth, and consist mainly of a tissue exactly comparable to dentine, capped with enamel, and continuous by its base with a mass of true bone, which takes the place of the crusta petrosa, or cement of the teeth.

A form of dermal exoskeleton, which is peculiar to and highly characteristic of fishes, is found in the fin-rays. These are developed in the integument either of the median line of the body, or in that of the limbs. In the former case, they usually enter into, or support, folds of the integument which are termed dorsal, caudal, or anal fins-according as they lie in the dorsal region, or at the extremity of the body, or on the ventral aspect, behind the anus. Ordinary fin-rays are composed of a hornlike, or more or less calcified, substance, and are simple at the base, but become jointed transversely, and split up longitudinally, toward their extremities (Fig. 6). Each fin-ray consists of two nearly equal and similar parts, which cohere bv their applied faces for the greater part of their extent; but, at the base of the rays, the halves commonly diverge, to embrace, or more or less completely coalesce with, cartilaginous or osseous elements of the exoskeleton. In the median fins, these are the interspinous cartilages, or bones, which lie between the fin-rays and the superior or inferior spines of the vertebrae. In the paired fins, they are radial or basal, cartilaginous or osseous, elements of the endoskeleton.

The Amphibia in general are devoid of dermal exoskeleton, but the Caecilicae have scales like those of fishes. Ceratophrys has plates of bone developed in the dorsal integument, which seem to foreshadow the plates of the carapace of the Chelonia; and the extinct Labyrinthodonts possessed a very remarkable ventral exoskeleton.

The Ophidia have no dermal exoskeleton. Many Lizards have bony dermal plates corresponding in form and size with the epidermal scales. All Crocodilia have such bony plates in the dorsal region of the body and tail; and in some, such as the Jacares and Caimans, and the extinct Teleosauria, they are also developed in the ventral region. In these animals there is a certain correspondence between the segments of the exoskeleton and those of the endoskeleton. But the dermal exoskeleton attains its greatest development in the Chelonia, and will be particularly described under the head of that order.

In the Mammalia the development of a dermal exoskeleton is exceptional, and occurs only in the loricated Edentata, in which the dorsal region of the head and body, and the whole of the tail, may be covered with shields of dermal bone.

In connection with the dermis and epidermis, the glandular and pigmentary organs of the integument may be mentioned. Integumentary glands do not appear to exist in Fishes, but they attain an immense development in some of the Amphibia, as the Frog. Among Reptilia, Lizards frequently present such glands in the femoral and cloacal regions; and, in Crocodiles, integumentary glands, which secrete a musky substance, lie beneath the jaw. In Birds they attain a considerable size in the uropygial gland; and, in Mammalia acquire a large development in connection with the sacs of the hairs, or as independent organs, in the form of sweat-glands, musk-glands, or mammary glands.

The color of the integument may arise from pigmentgranules, deposited either in the epidermis or in the dermis; and, in the latter case, it is sometimes contained in distinct chromatophores, as in the Chamaeleon.