AGNATHA

class of primitive, jawless fishes that contains the lampreys and hagfishes (collectively, cyclostomes) and some extinct groups. Living species of the Agnatha lack true jaws, paired appendages, and body scales, and their skeletons are cartilaginous. They breathe oxygen with paired gills and have muscular, nervous, sensory, circulatory, and excretory systems similar to those of lower fishes. The body of a lamprey is elongated and eellike. It has developed eyes, a nostril on the top of its head, a low dorsal fin, and a low caudal fin that extends around its tail. Lampreys use a disk of rasplike teeth to attach themselves to larger bony fish and feed off them parasitically. An anticoagulant released in their saliva aids in digestion. Breeding occurs in fresh water. Each egg hatches into a wormlike larva called an ammocoete, which burrows into the silt of the streambed and feeds on microscopic plants from the water. After three years the ammocoete undergoes metamorphosis, and upon reaching maturity the lamprey returns to marine waters to repeat the life cycle. The hagfish also has a long, eellike body. It lacks a dorsal fin, but the caudal fin extends to the dorsal and ventral sides of the body. Hagfishes have covered vestigial eyes and a nostril above their mouths; they seek food entirely by scent. They have a rasping tongue, but the mouth is surrounded by short tentacles rather than a sucker. Hagfishes usually burrow with their tails into sea bottoms, leaving only their heads protruding to scent prey; they generally feed on dead fish or soft-bodied invertebrates. The gonads of hagfishes contain both ovary and testes, but these fish are not hermaphroditiconly one of these organs will function. The eggs laid by the female hatch into small hagfishes. Fossil orders include Osteostraci, Anaspida, Heterostraci, and Coelolepida. Osteostraci had a flatter body than that of the lamprey or hagfish and a heavy bone shield around its head and gills. The body of anaspids was laterally compressed, suggesting that they were free-swimming rather than burrowing fish. The heterostracans were heavily armoured, with nostrils at each mouth corner, rather than the single nostril of lampreys and hagfishes. Little is known of the Coelolepida, an order that flourished in the Late Silurian Epoch (421 to 408 million years ago).