Fish are vertebrate animals, it is, they all have a vertebral column or ‘spine’. There are two main groups of fish, bony fish Teleosts and cartilaginous fish Elasmobranchs. As the average names imply, the skeletons of teleosts are made from bone while the elasmobranchs have cartilaginous skeletons. The elasmobranchs include sharks, rays and dogfish which differ from teleosts in many respects. The teleosts are much more numerous, with a better range of species than the elasmobranchs. All fish are aquatic and breath by soaking up dissolved oxygen in the water using their gills.