Cartilaginous Fishes, a designation which may be usefully restricted to that sub-class of fishes to which sharks and skates belong, and to which the technical titles of Elasmobranchii and Chondropterygii are usually applied. It is true that the double-breathing Dipnoi and many of the Ganoids are in great part cartilaginous, but there is no possibility of confusing either of these sub-classes with the Elasmobranchs. The primitive lancelet (Amphioxus), and the round-mouths, or Cyclostomata (Hagfish and Lamprey), are also cartilaginous, but these form classes by themselves, and in nowise deserve the title of fishes.
General Characters.—In these lowest fishes, then, the skeleton is gristly or cartilaginous, and the teeth and scales are (with the exception of slight hints in the vertebral column) the only bony structures. The skin bears skin-teeth (dermal denticles) or the so-called placoid scales, tipped with enamel, cored with tooth-substance or dentine, and based with bone. There is no gill-cover (operculum) over the gills, and the slits from the throat, usually five in number, open directly to the exterior. The gill-filaments are attached to either side of the partitions between the gill-slits. An anterior gill-slit, ahead of the usual five, usually opens on the dorsal surface behind the eye, bears only a trace of a gill, and is known as the spiracle. The ventricle of the heart has an anterior contractile portion, known as the conus arteriosus, which leads on into the vessel taking impure blood to the gills. The hind-fins are situated far back, and bear accessory clasping organs in the males. The intestine has a spiral fold running down it internally, known as the 'spiral valve.' The air-bladder is almost always entirely absent, but rudiments of it are sometimes seen. The eggs are usually laid in horny cases, such as the mermaid's purse of the skate; but not a few forms are viviparous—that is to say, the eggs develop in the oviducts. The embryos bear external gills. The Elasmobranchs are marine, and voraciously carnivorous in their diet.
History.—The cartilaginous fishes are undoubtedly the most primitive existing forms, and are of special interest on that account. The cartilaginous nature of their internal skeleton was obviously unsuited for fossil preservation, and thus their earliest records are limited to the bony scales and teeth. The oldest traces are found in the Upper Silurian, and are preceded by the armoured ganoid Pteraspis. They are scanty in the Devonian, but profuse in the Carboniferous, and become predominant among fishes in the Jurassic period. They retain this ascendancy throughout Tertiary times. A shark (Chlamydoselachus) from Japanese waters may be fairly ranked as the oldest living type of fishes. It has pronged teeth like the 'cladodonts' of the Middle Devonian, an anterior mouth, two dorsal nostrils, a large free opercular fold, six gill-clefts, and a slightly bent-up asymmetrical tail. It has been lately described by Garman (Bull. Mus. Comp. Zool. Harvard, xii.), and its ancient affinities invest it with peculiar interest.
Classification.—Of living forms about 285 species are known, referable to some 66 genera. A divergent offshoot from the main tribe is represented by the Holocephali (with a single gill-opening and jaws firmly fixed to the skull), and including the two genera Chimæra (King of the Herrings) and Callorhynchus. All the other members of the sub-class have 5 to 7 gill-openings and jaws movably attached to the skull. They are often designated
Plagiostomata, or cross-mouthed, for the mouth lies as a transverse slit on the under side of the head. They are conveniently distinguished into two sub-orders—(a) Selachoidae—shark-like forms, and (b) Batoidei—skate-like forms. In the former the gill-openings are lateral, and the body more or less cylindrical; in the latter the gill-openings are ventral, and the body flattened. Sharks, dog-fish, monk-fish or angel-fish, illustrate the first sub-order; skates, rays, torpedos, and sting-rays, the second.