SELACHIANS, a group of fish-like vertebrates that includes the sharks and rays. They are often ranked as a sub-class of the Pisces, but differ so fundamentally from the bony fishes that it is best to recognize them as a distinct class, Selachii, which may be thus defined :—Craniate vertebrates with jaws, and with branchial arches supporting the gills. Nasal organs, paired blind sacs, with incompletely divided external apertures. Skin with denticles struc turally similar to teeth ; no dermal ossifications. Internal skele ton cartilaginous. Median and paired fins with horny rays, and with skeleton typically of a series of cartilaginous rods. Vertebral column of the notochord and its sheath, neural and haemal arches, and intermuscular ribs ; no supra-neural arches or pleural ribs. No air-bladder.
Most living sharks are more or less spindle-shaped, with trans verse or crescentic mouth, generally on the under side of the head, and with a series of five to seven on each side in front of or partly above the pectoral fins. The end of the tail is provided with a fin above and below the upper and lower lobes of the caudal fin. Generally the end of the tail is upturned and the lower caudal lobe is more developed than the upper (heterocercal tail) ; in swift pelagic sharks with pointed fins the end of the tail is strongly upturned, the upper lobe is greatly reduced and the lower is correspondingly developed, but in the slower bottom living forms with rounded fins the end of the tail is but little up turned and the caudal lobes are more nearly equal. The rays are bottom-living forms with flattened head and body, ventral gill openings and large pectoral fins. The Chimaeroids live on the bot tom, and have a long tapering tail and large paddle-like pectoral fins. From the correspondence between habits and form of the fins in living selachians, it is certain that the Palaeozoic Clado mented into two principal pieces, basals within the body and radials in the fin ; each of the rods has a muscle on each side. A. study of the development shows that all the fins begin as longi tudinal f olds of the epidermis, at the base of which a mesenchyme plate develops ; next a double series of buds from the body muscles migrate into the fins, and later the cartilaginous skeleton differen tiates out of the mesenchyme plate. From their similar structure
selachus and Acanthodes, with broad-based, pointed fins and strongly heterocercal tail, were pelagic, but that Pleuracanthus, with long tapering tail and paddle-like paired fins, lived at the bottom. (For a general account of the principal living forms, and their habits, see SHARK, RAY.) Gill-arches and Jaws.—Selachians are of special interest in that they are in many respects the most primitive of the verte brates, and a study of their structure and development throws light on the origin of jaws, teeth and limbs. They differ from the Cyclostomata in their method of breathing, water being taken into the pharynx and expelled through the gill-clefts by the ex pansion and contraction of the pharynx. The gills are series of parallel strap-like projections from the anterior and posterior walls of the gill-clefts, which are separated by thin septa ; the inner ends of the septa are supported by cartilaginous arches that nearly encircle the pharynx, and are divided on each side into four seg ments ; the principal elements of these arches, epi-branchials above and cerato-branchials below, are directed backwards and meet at an angle when the pharynx is contracted ; above the epi-branchials are the pharyngo-branchials, and below the cerato-branchials the hypo-branchials join a median series of basi-branchials. The jaws of Selachians clearly represent the epi- and cerato-elements of an anterior gill-arch, enlarged and covered with teeth. In modern sharks and rays the jaws are supported by the enlarged epi-ele ment, or hyomandibular, of the arch behind it, or hyoid arch, which articulates with the otic region of the skull, but in the Chimaeroids the upper jaw, or pterygo-quadrate, is fused with the skull, and the hyoid arch is not modified for suspension.