Fish have relatively large eyes. Fish are shortsighted. Next to eye's there are nostrils to sense the fragrance. Usually the body covers skin equipped with mucous glands (mucus has a protective function and reduces friction while swimming) and scales (sometimes there are also bone plates or skin completelly not covered with anything). There is also Lateral line going along the fish body which senses water movements. Fins - membranous tissue is stretched to the rays of the bone - facilitate movement, body control and balance the fish. There are even fins (pectoral and pelvic) and odd fins (dorsal, caudal, anal). Adipose fin is present only in some species, and is usually devoid of rays. Tail of the fish is the main helm and drive.
Even if you have fish whose name and origin is unknown, the external structure of the body can provide us with many clues.
To breath fish is using oxygen dissolved in water. Water flows sequentially through the mouth, gills, where gas exchange takes place, and comes under the gill covers. Some fish need more oxygen than water contains so they adjusted to this environment by breathing atmospheric air. For this purpose fish is using swim bladder and an organ called the labyrinth. Swim bladder also is responsible for helping the fish to swin at the righ depth.
Sexual maturity is different for all fish species. Readiness to spawn is indicated by fish: change in coloration (bright colors), change in behavior (restless, belligerent towards other fish). In the majority of spawning females have sexual wart. Fertilization is usually external (oviparous fish) - male attracts a female to spawning nest where male spreads sperm on eggs deposited by the female. During internal fertilization (viviparous fish) inserts the male genital organ into female reproductive system. After a suitable time mature eggs come out into the water and straight away young fish comes out of the egg. The number of children and time to boarn is different for each species.
There are the following stages of fertilized eggs: