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.

 

-         Chrondrichthyes. All members of this group have real bone and their skeleton is made up of fleshy cartilaginous material. They are the predators of the parts of the world’s oceans in which they are located, using their sharp calcified teeth as weapons of attack. Included in this group are the various types of shark, rays, and skate comprising about 600 species in total.

 

-         Osteichthyes. Comprise the largest number of species by far with about 30,000 different species of these bony fish being known. These fish are commonly oval shaped but can also be spindly or flat. Most bony fish can remain buoyant due to having gas filled chambers located in their skeleton. This group can be further sp-lit into ray and lobe finned fish. The ray type has flexible skeletons while the lobe fish tend to be more muscular. Salmon, eels, and silversides are all species of bony fish.

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