Being members of the phylum Chordate all fish possess a main dorsal nerve cord, a skeletal rod notochord, and paired gill slits. Their habitat is primarily the ocean, but they can also be found in inland rivers and lakes.
The classification of fish found around the globe falls into three main categories: Agnatha (jawed fish), Chrondrichthyes (cartiliginous fish), and Osteichthyes (bony fish, which include both ray and lobe finned fish).
- Agnatha Fish. There are about 50 species of this type, which lack a jaw, paired fins, and any internal skeleton. In more ancient species they had bony skin plates which are no longer present in living species. The agnatha fish have up to seven gill pouches, but do not possess any form of stomach in their food digestive system. Both Hagfish and Lampreys are within this group.