Fishes
 Almost three-quarters of the Earth's surface is covered by water, in which live 20,000 known species of fishes. From their earliest jawless ancestors in the Silurian period, 450 million years ago, fishes have diversified to inhabit widely differing aquatic habitats. Despite their similar basic design, fish forms show considerable variety.
Fishes are vertebrates, with a backbone of either bone or cartilage. Among their distinguishing characteristics is the fact that they have fins, mostly in paired sets, and that their bodies are covered with scales. Most fishes breathe using internal gills, to which blood is pumped by a two-chambered heart (mammalian hearts, by contrast, have 4 chambers). They are poikilothermal ("cold-blooded"), because their body temperature varies according to the temperature of the water. Tropical fishes, therefore, do not have cold blood at all. Most fishes are oviparous (egg-laying), though in some species the eggs are retained in the female until after they hatch.
<== Salmon shark, Lamna ditropis
Fishes are divided into 2 groups: bony fishes (class Osteichthyes) and cartilaginous fishes (class Chondrichthyes), according to wether the main skeletal material is comprised of bone or cartilage.
Most fishes living in clear waters have good eyesight. Nostrils in the snout allow for the sense of smell. Sharks are renowned for their ability to detect blood in the water from as far as one-third of a mile. Many fishes that live in muddy water or are active at night also have an electric sense.
About 650 fishes of cartilaginous fishes have been identified. These are divided into two groups: the chimeras (subclass Holocephali), and the elasmobranchs (subclass Elasmobranchii).
There are about 25 species of Chimeras most of them are deep sea fishes. Chimeras differ from other cartilaginous fishes in possessing only one pair of gill openings. The upper jaw carries tooth plates and is fixed to the brain case, unlike the mobile jaws of sharks. Chimaeras have long thin tails and swim by flapping their large pectoral fins.
Elasmobranchs includes sharks, dogfish, skates and rays. All have gill slits, and usually in front of these, another opening called the spiracle.
Most sharks and dogfish have a streamlined, torpedo shape. The smallest members of this group are the dogfishes, many being smaller than 3 feet in length. Dogfishes live in shallow coastal waters, where they feed on mollusks, worms and other invertebrates.
Bony Fishes
About 95% of living fishes belong to the class Osteichthyes (bony fishes), making this the largest vertebrate class. Bony fishes can be readily distinguished from cartilaginous by the presence of a bony gill cover (operculum). Also, the mouth is usually at the very front of the head, instead of on the underside as in cartilaginous fishes.
Reproductive strategies also differ from those of cartilaginous fishes. In contrast to the large yolky eggs of the latter, bony fishes tend to lay numerous small eggs, the number (which can be millions), usually relating to the hazards of the life style. Because the males do not possess claspers to transfer sperm, fertilization takes place externally.
Source:
1. Mike Janson and Joyce Pope (consultant editors). The Animal World
2. CalPhotos Photo Database
3. U.S. Fish and Wildlife Service Multimedia database
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