Comparative Anatomy of the Integument
The microscopic bodies of protozoans are without a true integument, although in Amoeba there is a clearer marginal area, the ectosarc, which is different from the more granular inner part, or endosarc of the cell.
In other invertebrates that expose a cellular covering to the outside world, the integument is entirely ectodermal in origin, the mesodermal component being absent. No one has ever heard of leather being made from any backboneless animal, for leather is manufactured from the mesodermal part of vertebrate skin, and there is no such, thing as invertebrate corium.
The simple invertebrate skin is called hypodermis, in distinction to the epidermis and corium of the compound vertebrate skin. The hypodermis may consist of a single layer of flat epithelial cells, as in sponges and many coelenterates; of columnar epithelium, as in worms generally (Fig. 141); or of ciliated epithelium, as in flatworms and various larvae. Sensory and gland cells of various kinds may be interspersed between other cells of the hypodermis, and thus be in a favorable position to come into relation with the environment.
Frequently the hypodermis secretes a more resistant outer coat of chitin, lime, or other substance, that is not in itself cellular but which comes to constitute an integumental exoskeleton. This is particularly the case with arthropods and mollusks. As the body increases in size within the unyielding integumentary armor, it becomes necessary periodically for the hypodermis to loosen, and to cast off the lifeless, unaccommodating secreted envelope in order to renew it on a larger scale. Reminders of this process of ecdysis, or “molting the skin,” which is typical of arthropods particularly during the growing stages of metamorphosis, still persist even among vertebrates in the various ways by which dead corneal cells are sloughed off from the epidermis.
Unlike arthropods, most mollusks do not undergo general ecdysis but retain, with unfortunate parsimony, the exoskeletal limy shells secreted by the hypodermis, until they become so weighted down by adding layer after layer that locomotion is made difficult and sensation largely superfluous. Eventually sedentary contentment and accompanying degeneration take the place of the natural progressive evolutionary consequences that follow upon a more active and exploratory existence.
Among tunicates or ascidians, which occupy a borderland position between invertebrates and vertebrates, the epidermis is much like the hypodermis of lower forms, because of its power to secrete an external tunic of non-cellular material (Fig. 142). The peculiar substance secreted is called tunicin, which is not encountered elsewhere in the animal kingdom, although a chemically similar substance, cellulose, is a widespread constituent of plant tissues. Not only blood vessels and nerves but wandering irregular mesenchyme cells also penetrate into the tunicin matrix thus secreted, adding to the protective toughness of the mantle or tunic which gives these animals their general name of “tunicates”.
In amphioxus the typical compound integument of the vertebrates is reduced to its simplest expression. The epidermis consists of a single layer of columnar cells, which in the larval stage are ciliated as in certain worms, and which later produce a thin non-cellular cuticle that is reminiscent at least of the exoskeletal structures secreted by the hypodermis of invertebrate forebears. Thus amphioxus, in assuming the dignity of a vertebrate, does not entirely burn all its invertebrate bridges behind itself. The corium in the skin of amphioxus is represented by a thin layer of gelatinous connective tissue overlying the musculature (Fig. 143).
The slippery lampreys and hagfishes specialize in a highly glandular skin (Fig. 144). There are no scales present to restrict or modify the abundant and characteristic glands of various kinds, principally mucous, that are distributed among the cells of the thick many-layered epidermis. Epidermal cells in the skin of cyclostomes, from the deep-lying germinative Malpighian region to the surface, do not exhibit the same sort of progressive degeneration toward a lifeless corneal condition that is characteristic of the mammalian skin. The outermost cells even retain their youthful cytoplasmic character and are active enough to secrete a thin cuticle over their exposed surfaces, a lingering trace perhaps of long-vanished invertebrate days.
The horny teeth (Fig. 145), upon the surface of the pistonlike fleshy tongue and the wall of the buccal funnel, are the only epidermal cornifications in these animals. Periodically shed and renewed in the orthodox fashion of other corneal structures, they are thus to be regarded as corneal modifications of the epidermis.
The corium, which is thinner than the epidermis in these primitive aberrant fishes, is an interwoven network of vertical and horizontal connective tissue fibers, practically undifferentiated into strata.
The amphibian skin has much in common with that of cyclostomes, being highly glandular, scaleless, and with a relatively thin corium (Fig. 146). The epidermis, although consisting of several layers, is nevertheless thinner than that of cyclostomes. The glands, however, are of a more complicated type, being composed of several cells each, instead of a single cell as in cyclostomes. Although arising in the Malpighian layer of the epidermis, the compound integumental glands of amphibians do not remain in an epidermal position, as do the skin glands of cyclostomes, but push deeper down into the corium. Since amphibians are transitional animals, in and out of a water habitat, their plentiful glands help to keep the skin moist and sufficiently permeable for respiratory service. The vascularization of the amphibian skin is particularly pronounced during the critical period of metamorphosis, when, in some cases, the unusual vertebrate condition of penetration of the epidermis by capillaries takes place.
Among the higher amphibians which spend much of their time out of water, the corneum is differentiated in the epidermis with the result that ecdysis occurs, the dead outer layer sloughing off, sometimes in fragmentary rags and tatters. The corneum, however, is especially characteristic of land animals, not being as evident in aquatic forms.
A secreted invertebrate-like cuticle, such as amphioxus and cyclostomes have, is transiently present in some larval amphibians, much to the delight of the comparative anatomist, although it no longer appears in adult forms.
Pigment cells of the amphibian skin are located mostly in the corium, where they come under the control of the nervous system so that certain species, tree frogs for example, are able to adapt themselves with considerable success to the color of the background on which they find themselves, thus escaping detection. It should be noted that the skin glands of “warty” toads take on an irritating or even poisonous function, which discourages the advances of molesting enemies.
In many vertebrate species scales form a conspicuous modification of the integument. The character of the different kinds of scales will be considered later. In this connection attention will be directed simply to some of the characteristic integumental features of vertebrates with scaly skins.
Most fishes possess scaly skins. Aside from scales the integument of fishes is generally marked by the snugness with which it fits the underlying muscles. There is a tailored nicety about the skin of a fish that is not apparent in the baggy jacket of a frog, the loose integument of a bird, or the comfortable elasticity and wrinkles of the mammalian skin.
The epidermis of fishes is highly glandular. Usually the epidermal glands are superficial one-celled structures outside the scales (Fig. 147), which serve to anoint the body with mucus. Although prophetic indications of a corneum are found in some instances among fishes, in general the epidermis, as in cyclostomes, does not differentiate a definite external corneal layer, for a dead corneum is an adaptation to life on land and exposure to dry air.
The corium of fishes is a typical meshwork of connective tissue, more stratified in its deeper parts, and bearing the embedded scales to which it gives rise. Frequently the corium as well as the epidermis displays pigment of different kinds that decorates the body with an endless variety of patterns and colors, particularly in brilliant bizarre tropical fishes.
In the evolution of amphibians it appears that multicellular glands have displaced scales as the most characteristic features of the skin. These two structures are to a considerable degree mutually exclusive. A truly glandular skin would be hampered by the presence of scales, while a scaly skin is in no wise a convenient place for glands. The tiny one-celled mucous glands over the surface of the scales of a fish are not to be compared in this connection with the dominant many-celled glands that characterize the amphibian skin.
Extinct stegocephals of the Carboniferous Age were as scaly as any of their contemporary fishlike neighbors. Many of them were large creatures resembling salamanders in form though greater in size. They were conspicuously clothed with a cumbrous platelike armor quite in style, for they lived in the days of scales when defensive knighthood was in flower among the animals of the earth.
Of modern amphibians only the degenerate tropical caecilians (Gymnophiona) have any suggestive trace of scaliness. The cylindrical bodies of these small wormlike animals are encircled by bands of tiny scales embedded in the skin, alternating with areas of a typically glandular character (Fig. 148). In the skin of these lowly inconspicuous bearers of the amphibian name, is written the final episode of the evolutionary story of the rout of scales by glands.
The high water mark in completeness and elaboration of a scaly skin is reached by reptiles. One has only to examine with care the pattern, sculpture, and arrangement of the scales on a snake or a lizard, to be impressed with their exquisite perfection.
As a group, reptiles are definitely committed to life on land, in spite of certain backsliding exceptions. This fact has left its modifying impress on the skin, which is no longer thin, moist, and respiratory, but thick and cornified against exposure to dry air. The struggle for a place in the sun between scales and glands has had quite a different issue in reptiles than in amphibians, since the former habitually rub much against the dry ground, thus having use for a corneal skin to safeguard them against frictional contact as well as desiccation in dry air. In consequence ecdysis is necessary for the removal of the dead outer layer of epidermis. Integumentary glands, which are superfluous in a highly cornified skin, are found only in exceptional cases as relics of the days before the ascendency of scales.
Some extinct reptiles, for example ichthyosaurs and pterodactyls, apparently had a scaleless skin, but most of the dinosaurs and their mesozoic relatives were burdened with an enormously developed integumentary armor made up of large dermal plates (Fig. 36d), which were usually embossed in bas relief, and sometimes bore along the back formidable spines projecting upward two feet or more.
The corium in modern reptiles plays a secondary role, while the epidermis reaches perhaps a greater elaboration than in any other group of vertebrates.
Among birds and mammals scaliness is of exceptional occurrence. The scaly legs and feet of feathered birds (Fig. 149) reveal their reptilian ancestry, while there are a few scale-specialists among mammals.
Anyone who has ever attempted taxidermy knows how loose, thin, and easily torn is the skin of a bird. Those parts not covered by feathers, like the shanks and the bare areas around the base of the beak, exhibit a thickened corneal layer of epidermis, but everywhere else not only the epidermis but also the corium is reduced to a delicate thinness.
The typical looseness of a bird’s skin, so unlike the tightly fitting integument of the fishes, is an advantage in flight, enabling the muscles, unhampered by a binding integumentary covering, to contract freely and to change their shape easily. The looseness of the skin on the belly of penguins serves a special purpose adapted to icy antarctic conditions. During incubation the single egg is lifted off the frozen ground to a secure position on top of the webbed feet of the parent bird and a generous apron of loose skin from the region of the belly is snugly wrapped around the egg to keep it warm.
Exoskeletal structures of birds, such as feathers, beaks, leg-scales and claws, are entirely epidermal, since dermal elements like the scales of fishes or the bony plates of certain reptiles are absent in this group.
Among the few cases of real scaliness in mammals are the armored armadillos of America and the pangolins or scaly anteaters of Africa. The skin of the fetal brown bear and European hedgehog (Fig. 150) too are scaly all over with hairs interspersed. Rats, opossums, and beavers have scaly tails that are conspicuous emblems of ancient allegiances which the comparative anatomist who runs may read.
The essential features of the mammalian integument have already been described in the previous section on the human skin. It may be emphasized here, however, that among mammals the corium reaches its greatest development, becoming many times thicker than the epidermis.
The conspicuous modifications of the mammalian epidermis are hairs and glands. These structures with others will be considered in the next section.