Vertebrates
The vertebrates are the largest and ecologically most important subphylum of animalia. They do not include the largest number of species or largest number of individuals per species, but they do include the largest animals, whales, and the largest average-sized species. Only the largest trees exceed the mass of whales.
Characteristic of vertebrates is the possession of a vertebral column, a braincase, medial fins, a skeleton of cartilage and/or bone, a true skin (epidermis plus dermis), a kidney, liver, and pancreas, a specialized heart and blood cells, and a neural crest.
The vertebrates developed after the Protochordata (lacking cranium, vertebral column and specialized anterior sense organs) invaded freshwater. The radiation of the vertebrates to tetrapods occurred in freshwater. So marine lineages of vertebrates migrated from freshwater to the oceans secondarily.
The Subphylum Vertebrata consist of seven extant classes. There are several arrangments of the classes, not all resulting in groups that posses natural taxa. The most convenient arrangement is into 2 superclasses; the superclass Pisces (fishes in the broadest sense including Class Agnatha, Class Elasmobranchiomorpha and Class Osteichthyes) versus the superclass Tetrapoda (terrestrial tetrapods including Class Amphibia, Class Reptilia, Class Aves and Class Mammalia. One alternative is the Agnatha, or jawless fishes, versus the Gnathostomata, or jawed vertebrates. Another is the arrangement into the Amniota (including the reptiles, birds, and mammals) based on the possession of a complex set of extraembryonic membranes, as opposed to the Anamniota (including the fishes and ampfibians), which have only the trilaminate yolk sac.
(after Bock, 1982)