A jawless predator
Another larva provides evidence for the next step in the vertebrate evolution. This is the Lamprey, class agnatha (without jaw) which have larvae that are also jawless, blind and without fins except for a fringe around the tail and very similar to lancelets. These larvae were once thought to be adult creatures called ammocoetes. The adult lamprey is very fish-like except being jawless. It possesses the beginnings of a backbone in the form of cartilaginous elements. They also have a clearly defined head, with two small eyes, a single nostril leading to a blind sac, and on either side of the neck a row of gill slits. The mouth is a circular disk and possess a tongue with sharp spines. It is with this disk that the lamprey clamps itself on to fish which it parasitizes.
A lamprey is a jawless fish with a toothed, funnel-like sucking mouth, with which most species bore into the flesh of other fishes to suck their blood. In zoology, lampreys are not considered to be true fish because of their vastly different morphology and physiology.
Agnatha (Greek, "no jaws") is a paraphyleticsuperclass of jawless fish in the phylumChordata, subphylum Vertebrata. There are two extant groups of jawless fish (sometiems called cyclostomes), the lampreys and the hagfish, with about 60 species between them. In addition to the absence of jaws, Agnatha are characterised by absence of paired fins; the presence of a notochord both in larvae and adults; and seven or more paired gill pouches. The branchial arches supporting the gill pouches lies close to the body surface. There is a light sensitive pineal eye (homologous to the pineal gland in mammals). There is no identifiable stomach. Fertilization is external. The Agnatha are ectothermic, with a cartilaginousskeleton, and the heart contains 2 chambers.