Cartilaginous Fishes

Cartilaginous Fishes, a book name for an order of fishes (Chondropterygii - the Elasmobranchii of Bonaparte), of the sub-class Palaeichthyes (q.v.). The mere fact that the skeleton is cartilaginous is not sufficient to constitute a fish a member of this order; for in the Dipnoi and very many others of the Ganoids the skeleton is not ossified. On the other hand, Amphioxus (q.v.) and the Cyclostomata (q.v.), in all which the skeleton is cartilaginous in a high degree, fall considerably below the rank of fishes and form separate groups. [Chordata, Craniata.]

As the name of the class imports, these fishes date from a very remote period, and from the nature of the skeleton the remains are chiefly limited to the bony scales, teeth, and fin-spines. They range from the Silurian to the Jurassic, in which formation they exceed all other fishes in number, and this excess continues up to and through Tertiary times.

These fishes are nearly all marine. The skeleton is cartilaginous with traces of ossification in the vertebras of some genera. The vertebral column is generally heterocercal (q.v.), the upper lobe of the caudal fin produced, except in the true Rays. Median and paired fins are present, the hinder pair on the abdomen. The air-bladder is absent or quite rudimentary; the heart has a contractile arterial cone communicating with the vessel which returns the impure blood to the gills for aeration. Gill-cover absent; gills attached to the skin by the outer margin with a varying number of intervening gill-slits. In some genera a gill-slit bearing a rudimentary gill, known as the spiracle (but bearing no relation to the spiracle of the Cetaceans), is placed behind the eye. The intestine has a spiral valve. The skin bears calcified papillae, or bony scutes, to which the now obsolescent name of Placoid Scales was formerly applied. The ova are large and few in number, impregnated within an internal cavity, and in some instances deposited within horny cases which are often found empty on the sea-shore, and are locally known as mermaids' purses, fairy purses, etc. Some species are viviparous; that is, the eggs are hatched within the body of the mother. The males have intromittent organs attached to the ventral fins. The embryo is furnished with external gills, which fall off before maturity is reached.

The order is divided into two sub-orders: (1) Plagiostomata, or Plagiostomi, containing the Sharks and Rays; (2) Holocephala, containing only one family, of which the Chimaera (q.v.) is the type.