Chordate Classes I — Cyclostomata, Chondrichthyes and Osteichthyes

Class Cyclostomata (Greek kyklos, circle + stoma, mouth) comprises the most primitive living vertebrates, the only survivors of the jawless (Agnatha) vertebrate line. As the name suggests, the mouth is a circular, jawless, sucking or biting aperture, without true teeth (though the mouth may bear horny, tooth-like structures used for rasping). The body lacks paired fins and scales altogether, and the cranium and axial skeleton, though present, are entirely cartilaginous. Cyclostomes are either ectoparasites, attaching to a host fish and rasping its flesh and blood, or scavengers on dead and dying fish. Most species are marine but migrate into fresh water to breed (they are anadromous), after which the adult usually dies; the ammocoete larva that hatches lives, filter-feeding, in fresh water for several years before metamorphosing. Petromyzon (the lamprey) and Myxine (the hagfish) are the standard examples.

Class Chondrichthyes (Greek chondros, cartilage + ichthys, fish) comprises the cartilaginous fish — marine, streamlined predators whose entire endoskeleton, including the jaws, is made of cartilage rather than bone. Unlike the cyclostomes, chondrichthyans possess strong, functional jaws (usually studded with several rows of replaceable teeth) and paired fins. The mouth is characteristically located on the ventral surface of the head. The skin is covered by minute, backward-pointing placoid scales, which give sharkskin its characteristic rough, sandpaper-like texture. Gill slits are typically visible externally, without the protective operculum (gill cover) found in the bony fish, and, because chondrichthyans lack a swim (air) bladder, they must swim continuously or sink — a demand met by their oil-rich, buoyant liver and by the lift generated by their pectoral fins in motion. Fertilisation is internal, and many species are ovoviviparous, retaining fertilised eggs within the body until they hatch. Scoliodon (the dogfish), Pristis (the sawfish), Carcharodon (a large predatory shark) and Trygon (the stingray) are the classic examples. …