While every class of vertebrates except cyclostomes has some representatives with a scaly skin, the presence of scales may be regarded as the most notable modification of the integument of fishes and reptiles. Whenever fishes lack scales, as for example many Siluridae and certain bottom-feeding forms, it is to be regarded as a secondary modification and not the primary ancestral condition. Even in the case of the apparently naked eels, tiny vanishing scales appear for a time in the embryonic stages.
There are at least four general kinds of fish scales of particular interest to the comparative anatomist, namely, placoid, ganoid, cycloid, and ctenoid, not including bony dermal plates that reached a high degree of elaboration in extinct ostracoderms, and other armored fishes of early geologic times.
The most primitive fish scales are placoid, appearing first in the ancestral sharks of the Upper Devonian times, and found today among selachians generally. In structure a placoid scale consists of a somewhat flat, basal plate originating from the corium and embedded in it, and usually carrying a spiny projection of toothlike dentine capped with a harder substance considered by some observers to be enamel, formed by epidermal cells, but regarded by others as a special type of dentine, produced by dermal cells.
The obvious transition in both structure and position from placoid scales of the skin on the outside of the head, to the rows of teeth within the inner margin of the shark’s jaws is so continuous and unmistakable that teeth may be regarded as modified placoid scales (Fig. 169). The basal dermal plate of the scale corresponds to the root of a tooth, while the projection of dentine and its enamel cap are quite like similar familiar parts of a typical tooth. These relationships are not surprising to one who remembers that the lining of the anterior part of the digestive tract, formed from a stomodaeal invagination, is really modified epidermis.
Placoid scales in dogfishes and sharks are usually small and closely set without actually overlapping, although their backward-projecting enameled spines aid in effecting protection of the spaces of skin between the embedded scales. Pavement scales of various shapes may give added protection to the most exposed areas. Before the invention of sandpaper and emery cloth, the rasping “shagreen” skin of dogfishes and sharks, which is covered with sharp, thick-set placoid scales, was frequently used by cabinet makers for putting the final smooth finish on wooden surfaces. Quite large placoid scales equipped with jagged spines appear in skates and rays and are often localized in certain exposed areas, as down the median line of the back, leaving scaleless patches of skin unprotected.
Ganoid scales of considerable diversity are the common characteristic feature that stamps the ganoid fishes. The few genera of ganoids living in fresh waters today are the last survivors of a large populous and diversified order of fishes which once ruled the Devonian seas. Their scales furnish a variety of form and structure out of all proportion to the number of species involved. In the sturgeon, Acipenser, for example, they are large isolated bony scutes, not entirely covering the skin but located in exposed situations on the body where there is the greatest wear and tear, like the rows of brass-headed nails decorating the edges of great-grandfather’s chest.
In the garpike, Lepidosteus (Fig. 170), on the other hand, as well as in the related forms Calamoichthys and Polypterus, the scales are hard polished rhombic plates fitting edge to edge, or very slightly imbricated one over the other, thus forming a complete armor. The skin of the spoonbill sturgeon Polyodon is almost entirely without scales, while in Amia modified ganoid scales occur on the head but only cycloids in the trunk and tail regions.
The scales of fossil ganoids are quite large and platelike, just as in living survivors of the group where scales that cover the head sometimes become enlarged into dermal scutes which take part in the formation of the investing skull bones.
The outer surface of scales of this type is composed of ganoin, a hard shiny substance secreted by the corium, not at all homologous with the ectodermal enamel that caps placoid scales. This material is not present, however, on the ganoid scales of Acipenser, Scaphirhynchus, and Amia. The main underlying part of ganoid scales is made up of isopedin, a connective tissue substance in which bone cells are embedded and which like bone is penetrated by Haversian canals containing capillaries.
Louis Agassiz distinguished two kinds of scales in teleosts, which he named, cycloid and ctenoid. Cycloid scales, as the name indicates, are rounded in shape (Fig. 171) and thicker in the center, thinning out towards the margin. They overlap like shingles and if spread over a surface like ganoid scales edge to edge would much more than cover the body. They are, however, embedded in pockets of the corium with only a part of the outer smooth margin exposed. Since they project diagonally at an acute angle with the surface of the skin, and overlap their neighbors, the entire body is protected by at least a double thickness of scaly armor at every point. Present-day bony dipnoans as well as Amia and some teleosts have cycloid scales.
Ctenoid (“comblike”) scales, also rounded in form, have in addition projecting teeth on the surface of the exposed areas (Fig. 172). All intermediate types between the cycloid and the ctenoid stages are to be found. In some fishes, as for example certain flounders, the scales on the upper side are ctenoid, while those on the under side are cycloid. Of these two types the cycloid scale is the more primitive, occurring first in the fishes of the Jurassic Period, while the ctenoid type did not appear until Cretaceous times.
Both cycloid and ctenoid scales are entirely dermal in their origin. The scleroblasts, or scale-forming cells in the corium, lay down two layers of different substance in the formation of a scale. The outer layer is homogeneous and bony, while the under side is fibrillar and contains calcareous deposits. Such scales increase in thickness and area by the activity of the scleroblasts, successive additions being indicated by concentric lines of growth like similar rings of growth exhibited in the cross section of a tree trunk. Inasmuch as periods of growth alternate with comparative inactivity in the case of most fishes, according to the seasonal variation of their food supply, it is possible to estimate the age of a fish by an examination of the diary-like lines on its scales.
In addition to marginal lines of growth, certain radial grooves are also present, caused by the failure of the outer homogeneous layer of the scale to be deposited in these places. Such radial grooves add somewhat to the flexibility of the individual scale, a very desirable feature since teleost scales, although thinner than the edge-to-edge ganoid type, by reason of their shingling arrangement, form a double envelope over the underlying muscles that might hamper free movement.
Some teleost fishes, for example, the pipefish Syngnathus, and its curious relative, the seahorse Hippocampus, do not have overlapping scales but instead are encased in a cuirass of bony plates.
Scales in all modern amphibians are absent, except for the tiny bands of scutes embedded in the skin of the tropical legless caecilians. The extinct stegocephalans that flourished in Devonian times were characterized by bony plates in the skin, particularly on the ventral side of the body.
Reptiles are represented today by only a few divergent specialized types, namely, lizards, snakes, turtles, crocodiles, alligators, and Sphenodon. Superficially quite unlike each other these survivors agree, so far as scales are concerned, in the predominant part that the epidermis plays in their formation. Small dermal plates, or ossicles, sunken in the corium and spaced with much regularity, are present in most reptilian species, although many lizards and snakes lack them. When present they are covered over by a continuous layer of epidermis in which the dry outer comeal part becomes thickened or embossed wherever it covers an ossicle. It is thinner and more flexible between the thickened parts or scales of the epidermis, thus allowing some freedom of movement (Fig. 173). Even in a skin without ossicles the corneum thickens into epidermal scales that hang together in a continuous sheet. Thus they form an unbroken armor in which the scales cannot be scraped loose, as may the separate independent dermal scales of teleost fishes.
As already mentioned the entire dry corneal part of the skin of snakes periodically loosens and is cast off. Such is not the procedure with turtles and alligators, for in these animals the corneal scales that overspread the dermal plates in the skin are discontinuous and the demands of increasing size are met by concentric marginal increments of growth which are added to each scale. The highest development of this underpinning of ossicles is reached in the armored turtles, although some of the extinct dinosaurs were “fearfully and wonderfully made” with respect to dermal scales.
Many reptiles, like the “horned toad” Phrynosoma, and Sphenodon and the alligators and crocodiles, exhibit spines or embossed patterns on the epidermal scales, while the head of a snake, which has to poke about between obstacles, is entirely ensheathed by large platelike scales, with the sense of touch that is thereby excluded from the surface of the head transferred to the delicate protrusible tongue.
There is very little to say about scales in the skin of birds, since aviation has no use for such heavy clumsy structures. Only on unfeathered areas, such as the shanks and around the base of the beak, is the epidermis thickened and cornified into a semblance of reptilian scales, and nowhere do dermal ossicles occur under corneal thickenings.
Scales, although generally replaced by hairs in mammals, still persist in a number of instances. Mammalian scales are epidermal like those of reptiles.
They are large horny and imbricated in the scaly anteater, Manis, although absent on the ventral side of the body. During ecdysis they are shed and renewed singly. In armadillos no ecdysis occurs but growth is accommodated within the shieldlike armor by marginal accretion of the separate scales as in turtles. Areas of bare skin between the rows of scales permit these animals to roll up into a ball for the purpose of defence. Their fossil forebears, the giant glyptodons of South America, could not roll up, but their thick fused scales made such an impenetrable barrier, like that of a war tank, that they must have been practically carnivore-proof. The fact that telltale hairs project between the scales of armadillos proclaims them to be mammals, although bearing a “wolf’s clothing” of scales.
A great variety of mammals have scaly tails reminiscent of earlier forebears, the beaver, rat, mouse, opossum, mole, shrew, and certain lemurs. Scales even appear on the back of the paws of moles and shrews, and it will be remembered that the fetuses of the brown bear and European hedgehog testify to some sort of a scaly pattern in the past by developing transient useless embryonic scales over the back. Only the armadillos have bony plates beneath their scales.