Ichthyopsida in which the epidermis may develope glandular ce/ls, but not glands in the ordinary sense of the term, and the cutis is devoid of muscular elements. The exoskeleton is greatly developed, and consists always of a dermal portion (bone or dentine), to tvhich enamel derived from the epidermis may be added. It takes the form of teeth, scales, or large scutes.
Epidermic sensory organs are largely developed. The azygos fins are supported by fin-rays, as also are the borders of the paired limbs which have the form of fins. There is no Eustachian tube in the proper sense, no tympanic membrane or cavity, nor stapes in relation with the ear. The alimentary canal terminates very generally in a cloaca, or else the anus lies in front of the genito-urinary apertures. There is no vena cava inferior. An allantoid outgrowth of the digestive tract is invariably absent.
The epidermis consists of polygonal cells in Elasmobranchii. It never developes a stratum corneum, but in some Teleostei and the Dipnoi the superficial layer of cells has a striated cuticular border. Flask-shaped mucous cells with the narrow end internal are very generally present, and in large numbers in some Teleostei and the Dipnoi. They occur in Acipenser (Ganoidei), but not in Elasmobranchii. They appear to rise to the surface and there burst. Branched cells, both pigmented and non-pigmented, are found both in the epidermis and the cutis. They are connected with nerve-filaments. Some Fish, e. g. Turbot and Trout, have the power of altering their colour in relation with the nature of the bottom on which they are living. The epidermis at the breeding season often assumes a silvery appearance, and in some instances a brilliant coloration, e.g. Stickleback (Gasterosteus). The epidermic sensory structures are nerve-eminences and end-buds. The former are rarely absent altogether (Balistes); they may be generally distributed over the body, e. g. Mugil, or more or less limited to certain lines.
These are typically, on the head a supra- and infra-orbital, and a mandibular line on each side, with a connecting supra-occipital line; on the body a lateral line, which follows the plane of division between the dorso- and ventro-lateral muscles, and more rarely a second line placed more on the dorsal aspect, and a third nearer the ventral surface. Scattered eminences may occur at the same time, e. g. Pike (Esox). They either project freely above the surface, or are sunk in grooves of the scales or in integumentary pits when scales are absent, e. g. Silurus glanis; or finally are lodged in canals which open externally from spot to spot. Echinorhinus spinosus and a few others among Sharks and the Holocephali have exceptionally an open groove in the region of the trunk. Canals are generally developed on the head, where they may be much branched as in Elasmobranchii; on the body also, but either only anteriorly, or posteriorly (some Teleostei), or throughout its whole length. The canals of the body are much branched in the Rays. The whole canal system and integumentary pits are filled with mucus secreted by certain of the lining cells.
This mucus is gelatinous and firm in Elasmobranchii, softer in other Fish. Elasmobranchii have peculiar structures known as ampullae on the head, and Ganoidei nerve-sacs in the same region. The latter are minute depressions with sense-cells at the bottom, beneath which lies a space filled by jelly. The former are tubular, and the base of the tubule is dilated and either simple or sacculated, and contains sense-cells in the lining epithelium. The ampullae are filled with gelatinous mucus. End-buds are as a rule plentifully scattered over the head and body in Teleostei, but they may be wanting entirely, e. g. in Mugil and Esox, where scattered nerve-eminences occur. They have only been detected in Torpedo among Elasmobranchii. But in all Fish with few exceptions, where they are replaced by nerve-eminences, they occur in the mouth and on the branchial arches. The sense-cells of the buds differ from those of nerve-eminences in extending from the surface to the base of the epidermis. End-buds are also generally prominent.
Curious 'eye-like' organs are imbedded in the skin of certain pelagic and deep-sea Teleostei (Scopelus, Chauliodus, &c). According to peculiarities of structure these organs are divisible into 'eye-like organs proper,' 'pearl-like/ and 'phosphorescent' organs. They all agree in possessing an iridescent layer resembling a tapetum, and like it reflecting light. The mucus coating the surface of the body is often phosphorescent, and some Fish possess phosphorescent organs, e. g. an appendage beneath the head in Eustomias.
The exoskeleton is rarely absent, e. g. in many Eels and all electric Fish; it is sometimes rudimentary. The primitive form occurs in Elasmobranchii in the shape of dermal teeth (= placoid scales), similar in structure and development to oral teeth and borne upon a plate of bone formed in the cutis1. The Siluroidei among Teleostei have dermal teeth also, moveably articulated, however, to their bony supports, and these supports often fuse into large plates. In other Fish teeth tend to disappear, and the supporting bone constitutes the scales, scutes, spines, or shields of the exoskeleton. Teeth, however, are found on the skin between the mandibles of Lepidosteus, and as minute spines on the edges of its scales. The enamel, however, is homogeneous, the dentine tubular and a pulp-cavity absent. Minute denticles of non-tubular dentine are also found on the outer surface of the scales of Protopterus (Dipnoi). The spines projecting through the epidermis from the scales and scutes of Acipenser are homogeneous structures. The scales of Lepidosteus and Polypterus among living (as of many extinct) Ganoidei are covered with a layer of homogeneous enamel. They are lodged, as are the scales of Teleostei and Dipnoi, in sacs of the cutis. The scales thin out at their edges and vary much in size.