Fish

Flying gurnard (top) and common skate embryos (bottom)The generic name fish is used to describe a very wide range of animals. Fish can be divided into:

  • chondrichthyes - cartilaginous fish, rays and sharks etc.
  • actinopterygii - ray-finned fish
  • sarcopterygii - lobe-finned fish, including lungfish, coelacanths and terrestrial vertebrates.

Cartilaginous sharks first appeared in the fossil record 500 million years ago and are considered to be the most primitive form of fish. They lack bone, although their teeth and parts of their vertebrae are strengthened with calcium.

The ray-finned fishes first diverged from the lobe-finned fishes around 400 million years ago and it is now estimated that there are around 27,000 species. Actinopterygians have a ray fin formed by a number of parallel bony rays controlled by the body musculature. Other defining characteristics include scales, jaw structure and swim bladder function. 

Sarcopterygian fish have a lobe in their fins containing bones and muscles. The fins are very flexible and can support the animal's weight on land, as in lungfish and tetrapods. Tetrapods include all mammals, reptiles, amphibians and birds.

 

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