Chordata: Classes Myxini and Hyperoartia

Classes Myxini and Hyperoartia

The jawless fishes, traditionally considered a single class, Agnatha, are the oldest known vertebrates. The only surviving members of this group are the hagfish (class Myxini) and lampreys (class Hyperoartia). These two classes may be grouped together in the superclass Cyclostomata. Cyclostomes have long, slender bodies with dorsal, ventral, and caudal (tail) fins, all in the median plane. Although in their lack of jaws or paired lateral appendages they represent a very primitive stage of vertebrate development, the modern cyclostomes are highly adapted for their particular ways of life. The hagfish is a specialized scavenger, and the lamprey is a parasite on other fishes. The hagfish has a rudimentary skeleton, of cartilage rather than bone, with a braincase, but no jaw. Its circular sucking mouth has rows of horny teeth. There is a single median nostril and the eyes are poorly developed. The lamprey has a round mouth without skeletal supports, a rasping tongue, and a single, dorsally located nostril. The lamprey has a few cartilaginous blocks around the notochord that constitute the bare rudiments of a backbone; a cartilage framework supports the gill region, and there is a rudimentary cartilage braincase. In cyclostomes, as in all fishes, water is taken in through the mouth and expelled through the gill passages; as water passes over the thin-walled gill filaments, dissolved oxygen diffuses into the blood, and carbon dioxide diffuses out. Throughout life, hagfish and lamprey retain the notochord, a supporting structure found in other vertebrates only in the embryo; they lack sympathetic nervous systems, spleens, and scales. The extinct relatives of the cyclostomes, called ostracoderms, were jawless fishes with bony armor and in some cases a well-developed bony skeleton.

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