It is an accepted convention to visualize the classification of the animal kingdom as starting with the Protozoa, the single-celled animals typified by the amoeba and ending with the mammals, which includes the human species. This resulted in a catch-phrase, less used today than formerly ‘from amoeba to man’.
When set down on paper in tabular form, such a classification gives the impression of a steady and continuous progression from the Protozoa, through the lower invertebrates and the higher invertebrates to the vertebrates. It must be nearer the truth to suppose that somewhere among the more organized echelons of the lower invertebrates there came a parting of the ways. One path led to the higher invertebrates, the other to the vertebrates.
The earliest fossils of fishes have been taken from rocks of the Ordovician period, 450 million years old. They are recognizable as being fishes, though they lacked jaws. Relatively few species of these jawless fishes have been discovered and fewer still have survived. Form this ancestral stock, however, sprang two main lines of descent: the cartilaginous fishes typified by the sharks, skates and rays, which were almost entirely marine, and the bony fishes, represented by the more familiar fishes in our present-day rivers and lakes as well as many others in the sea.
Fishes were the first backboned animals to appear on earth, and they form by far the largest group of vertebrates. Unlike the other major divisions of vertebrates however, fishes are not a natural group; instead, they are an informal collection of 4 classes that are only distantly related to one another.
A typical fish breathes using gills, has a body covered with scales, manoeuvres using fins, and is cold-blooded. Most species live in either freshwater or the sea, but a few species move between both environments.
Giant Bizarre Fish
