Vertebrata

Collier's New Encyclopedia (1921)

in zoölogy, a division of the animal kingdom, instituted by Lamarck, comprising animals in which the body is con composed of a number of definite segments, arranged along a longitudinal axis; the nervous system is in its main masses dorsal, and the neural and hæmal regions of the body are always completely separated by a partition; the limbs are never more than four in number; generally there is a bony axis known as the spine or vertebral column, and a notochord is always present in the embryo, though it may not persist in adult life. A specialized hæmal system is present in all, and in all but Amphioxus there is a heart with never less than two chambers, and in the higher vertebrates with four. The vertebrata are usually divided into five classes: Pisces, Amphibia, Reptilia, Aves and Mammalia, and many attempts have been made to gather these classes into groups. One plan is to divide them into branchiata (fishes and amphibians), because at some portion of their life they are provided with gills, and abranchiata (reptiles, birds and mammals), having no gills. The latter are sometimes called amniota or allantoidea, because the embryo is provided with an ammon and an allantois, while both these are absent in the branchiata, which are therefore called anamniota or anallantoidea. Owen made two sections: Hæmatocrya, or cold-blooded vertebrates (fishes, amphibia and reptiles), and Hæmatotherma, or warm-blooded vertebrates (birds and mammals) mammals); and Huxley three: Ichthyopsida (fishes and amphibia), sauropsida (reptiles and birds), and mammalia. A later classification is to treat all the vertebrata as a division of a larger group, Chordata, distinguished by (1) temporary or permanent possession of a rod (the notochord) underlying the central dorsally-placed nervous system; and (2) the temporary or permanent presence of visceral clefts. The Chordata are divided into three groups: (1) cephalochordata, in which the notochord, pointed at the extremities, extends from one end of the body to the other; (2) urochordata, and (3) the true vertebrata, or craniata, in which the anterior end of the central nervous system is enlarged into a brain, and which becomes surrounded and protected by a cartilaginous capsule or skull.