Cyclostomata

Mouth of the sea lamprey ( Petromyzon marinus ) attached to an aquarium glass.

The cyclostomes ( cyclostomes ) are a superclass of chordates ( Chordata ) in which the still living jawless vertebrates, the hagfish ( Myxinoidea ) and lampreys ( Petromyzonta ), are combined. In total there are about 100 species of cyclostomes, hagfish 67 and approximately 40 Lamprey.

Hagfish worldwide live in all oceans except for the Red Sea and the Polar seas. In tropical seas they are exclusively inhabitants of the deep sea. Lampreys inhabit marginal seas and freshwaters in temperate zones; the majority of species in the northern hemisphere of the earth, and four more in the south east of Australia, in New Zealand and southern Chile.

Features

All lampreys have an eel-like, elongated and scaleless body with a cartilaginous internal skeleton and have no paired fins and the associated belt formations ( shoulder girdle and pelvic girdle ). The ability to form bone is missing. The most important element of the axial skeleton, the notochord that remains pliable throughout life. The spinal cord is flattened at the lampreys but less strongly. The neurocranium ends with the labyrinth and is without occipital lobe, the nervous system has no myelin sheaths.

A branchial is formed only in the lamprey. In both groups, the gills pockets are within the gill arches and the gill lamellae arise from the endoderm in the jaw mouths ( gnathostomata ) from the ectoderm and the gills pockets are outside the gill arches.

The name cyclostomes ( cyclostomes ) was awarded because of the round, jawless mouths of both taxa. The mouths have Hornzähnchen, and may be an adaptation to the parasitic lifestyle. A so-called tongue apparatus takes in both taxa the rasping food particles. Whether this Raspelapparat is homologous in both groups is not yet clear.

System

The superclass of cyclostomes ( cyclostomes ) was established in 1806 by the French zoologist André Duméril. With the extinct, armored Ostracodermi they were united in the group of jawless fishes ( Agnatha ). With the advent of the principles of cladistics sat down in the late 1970s, the view through that it must be in the cyclostomes a paraphyletic taxon and that the lampreys are more closely related to the jaw mouths ( gnathostomata ) than with the hagfish.

Molecular biological studies in recent years show that the cyclostomes are monophyletic but, that lampreys and hagfish have a recent common ancestral form, from which, however, shows no other group. So they share four unique miRNA families and 15 unique Para logia between primitive microRNA families.

The lampreys are said to have evolved from a common ancestor of all vertebrates in the Cambrian period about 500 million years ago, which was, however, considerably more complex than the cyclostomes. The cyclostomes then went through a degeneration and lost many, the typical features of vertebrates. 360 million years old fossils of lampreys and 300 million years old from hagfish are the modern forms quite similar.