This is the largest and most diverse group of fishes. Main traits, which may however be absent in some species, are are the
bony skeleton, swim bladder, and highly derived skeleton of the skull and the tail, allowing for protrusion of the jaws and
flexible caudal fins, respectively. External identification characters are: Fins usually are supported by rays. Scales,
if present, are ganoid, cycloid or ctenoid. There is no spiracle. Interopercle and branchiostegal rays are usually present.
The nostrils are relatively high up on the head.
Main traits of this group are cartilaginous skeleton, placoid scales, internal fertilization with claspers in males, serial
replacement of teeth, multiple gill slits, no lungs or gas bladder but instead large, buoyant livers, and a spiral valve intestine.
External identification characters are: Five to seven separate gill openings on each side of the head, the first often modified as
a spiracle. Dorsal fins and spines, if present, are rigid and can not be folded.
Main traits of this group are a cartilaginous skeleton, internal fertilization with claspers in males, no lungs or gas bladder
but instead large, buoyant livers, and a spiral valve intestine. All chimaeras lay large eggs with a horny shell and development
of embryos is direct, without a larval stage. External identification characters are: A single gill flap covers four internal
gill openings, and there is no spiracle. The first dorsal fin, with its poison-laden spine, is erectable. Skin in adults is
naked except for small dermal denticles along the midline of the back and on the claspers of the males. Males often have an
additional clasping organ on the head.
This group shares a common ancestor with the tetrapods and includes six lung fishes and two coelacanths. External identification
characters are: Paired fins are present and form lobes or filaments. The tail is heterocercal and there is a double dorsal fin.
Scales are cosmoid and of moderate to large size.
Main traits of this group are the lack of jaws, presence of functional eyes, and presence of a dorsal fin. Lampreys produce many
small eggs and females die after reproduction. External identification characters are: Single medium nostril opening present
between the eyes. Seven pairs of external lateral gill openings. The body is naked and eel-like. There are no paired fins.
Main traits of this group are the lack of jaws, lack of complete eyes, lack of true stomach, and skin respiration in addition
to gills. Hagfish are marine scavengers and can produce large amounts of mucus or slime. Hagfish spawn repeatedly throughout
their lives and produce few large eggs each time. The blind, toothless ammocoete larvae typically burrow in the bed of silty
streams. Many adult hagfish are parasitic on other vertebrates. External identification characters are: Eel-like body without
scales and without paired fins. There is no dorsal fin, only a confluent caudal fin.