Kingdom Animalia; Phylum: Chordata (Chordates), Subphylum: Vertebrata (Vertebrates)
Class: Bony Fish (Osteichtyes, Pisces)

The exact number of fishes cannot be specified. There are approximately 22 000 different species and scientists are still sorting them out.

Anatomy: Fishes are superbly designed for the water life while water covers about 71 % of the earth’s surface. Their skin is mucous and it is made of scales whose design and shape vary a lot. The majority of fishes have got a skeleton made of bones. The spine is made of vertebrae. Ribs are made from the lateral vertebrae prominences. These ribs stabilize the ventral side and protect inner organs as well. There are tiny bones being created in the paired fins area (breast and ventral fins).
From the evolutionary point of view, these small bones are a base for future extremities. Apart from that, the breast fins are a base for future shoulder-blade and the ventral fins are a base for future pelvic area. It is obvious that neither the shoulder-blade area nor the pelvic area are fixed to the spine. Both of them are fixed only to muscles and ligaments.

Physiology: The fishes spend their life in the water. They breathe with gills which contain a number of tiny blood vessels and capillaries. These are well protected. Heart of a fish has got two parts: an atrium and a ventricle. The blood from the ventricle flows to an artery which divides into the small ones. These small arteries bring blood to the gills where the blood is oxygenated. Then it flows to the spine artery which divides and subsequently brings blood containing inevitable oxygen to particular organs. Deoxygenated blood from all organs is then collected from all veins to the main one. Last but not least the main vein brings deoxygenated blood to the atrium. This is how blood simply circulates in the fish’s body. Few fishes have got primitive lungs.
Digestion is simple as well. Fish’s mouth may contain a huge variety of teeth which kill a prey and crush it. Then it follows to the pharynx and esophagus (oesophagus). Distal/Aboral oesophagus part is wider and it is followed by a stomach (gaster, ventriculus). Intestine (intestinum) is not very differentiated. In the ventral esophagus area there is a sack filled with gas. The gas pressure can be changed.
The brain consists of five parts. The most peculiar thing about fish is definitely the “side-line”. It is located on the lateral side of the fish’s body. It contains a number of sensory and very sensitive nerves. These nerves enable fish to orientate themselves when it is dark and in murky water. Other sensory organs include eyes, smell organ etc.
We can divide fishes into several categories. In the first category there are fishes which live in the fresh water (for instance Lepidosiren). They are able to breathe either with gills or with lung sacks as well. The second category contains extinct fishes. Latimera chalumnae is probably the only surviving fish at this time. The third category is a huge one. It contains fishes which inhabits seas and oceans but freshwater as well. Their fins are made of ray-shaped bones. In Europe we can find for example: Cyprinus carpio (Carp), Tinca tinca, Chondrostoma nasus, Leuciscus cephalus, Salmo (Salmon), Essox (Pike), Barbus barbus, Perca fluviatilis and so on.
The size of the fishes varies a lot. For instance The Whale shark measures up to 19 m whereas the Luzon Goby only 11 mm. Eating habits are different as well. Some fish feed on meat (a number of sharks, piranhas) whereas other ones feed on plankton, insects etc. (carps). Some fish inhabit sea (rays) and on the other hand there are only freshwater fish such as trout. Apart from that for instance salmons inhabit both types of the water throughout their lives.
To put it in a nut shell fishes represent a huge variety of animals which total number still remains unknown and moreover it exceeds the number of mammals, birds, amphibians and reptiles together.