Skin Glands.—The skin glands of the Cyclostomata (hags and lampreys) and fishes are generally unicellular and secrete slime which protects the surface of the body. Some of the teleostean fish have poison glands at the bases of their dorsal fins and opercula.
In the mud fish (Dipnoi) and amphibians multicellular spheri cal glands appear as involutions of the ectoderm. Sometimes, as in the so-called parotids of the toad, these form large masses. Reptiles and birds are singularly wanting in skin glands, though the latter have a large uropygial gland at the root of the tail which secretes oil to lubricate the feathers ; it is the chief constitu ent of the "parson's nose" of the fowl. In mammals, except the Cetacea, the sebaceous and sudoriparous glands already described in man are found ; some of the former sometimes attain a large size, as in the interdigital gland of the sheep, MUller's gland at the back of the pig's knee and the suborbital gland of ruminants. In addition to these, special scent-producing glands are often found in different parts, the most remarkable of which, perhaps, are the scent glands beneath the tail of the skunk, while in male monotremes there is a special poison gland in the leg which is connected with a spur in the foot.
Pigment.—Pigment cells are present both in the dermis and epidermis of fishes and amphibians, and the pigment may be either intra- or extra-cellular. In many cases it is under the control of the nervous system, so that forms like the flat-fish and the common frog can adapt their colouration to that of their background. In animals permanently excluded from the light, pigment is absent. In reptiles movable pigment cells are often found, as in the chameleon, while in birds the pigment is some times of great brilliancy in the necks and wattles. In mammals, as in man, the pigment is confined to the cells of the stratum mucosum layer of the epidermis.
Scales.—In the elasmobranch fishes scales are found composed of enamel superficially, and of dentine and bone deeply. They are developed from the epidermis and dermis, and in almost every way resemble the teeth of these animals, which are only modifi cations of them. The bony basal part of each scale is plate-like,
hence this kind of scale is known as placoid. In the ganoid fishes, such as the sturgeon, much larger plaques called ganoid scales form a complete armature. In the teleostean fishes the scales overlap like tiles and are either cycloid, having a smooth border, or ctenoid, in which the free posterior border is serrated. Existing amphibians are usually remarkable for absence of any skin armour, though in fossil forms (Stegocephala) it was very complete. The reptilian class is specially noticeable for the production of epi dermal scales, which undergo many modifications. In the Ophidia they are cast off periodically in one mass as the snake's slough, while in the Chelonia they form the different varieties of tortoise shell. Bony structures, developed in the dermis, may underlie these epidermal horny thickenings, and are very strongly devel oped in the dorsal and ventral bony shields of the Chelonia (cara pace and plastron), which secondarily fuse with the true endo skeleton. The armadillo is the only mammal which has a true bony exoskeleton.
Feathers.—Birds are remarkable for the possession of feathers, which are highly modified scales. The embryonic or down feathers are simple, and consist of a brush of hair-like barbs springing from a basal quill or calamus. From the whole length of each barb a series of smaller barbules comes off like branches of a shrub. The adult or contour feathers are formed at the bottom of the same follicles which lodge the down feathers and, by their growth, push these out. At first they are nothing more than enlarged down feathers, but soon one of the barbs grows enormously, and forms a main shaft or rachis to which the other barbs are attached on either side. From the sides of the barbs grow the barbules, just as in the down feathers, and these, in the case of the large wing feathers (remiges) and tail feathers (rectrices), are connected by minute hooks so that the feather vane, as opposed to the shaft, has a more resistant texture than it has in the feathers of the back or breast. The bird's moult is comparable to the casting of the scales in the reptiles.