Mammals are a quite well known group, however estimates for the numbers of described species still vary considerably, ranging from 4 300 in Biodiversity: the UK Action Plan (Anon. 1994), through 4 630 ...

Fish are cold-blooded aquatic vertebrates with gills, swim bladder, and paired fins. ©mnoor/Shutterstock.com A fish is a cold-blooded aquatic vertebrate with gills, swim bladder, and paired fins.

Taxonomists typically group fish into three superclasses: cartilaginous fishes (Chondrichthyes), bony fishes (Osteichthyes), and jawless fishes (Agnatha). Cartilaginous fishes, as the name suggests, ...