VERTEBRATA, one of the main subdivisions or phyla of the animal kingdom, including such familiar animal types as mammals (including man), birds, reptiles, amphibians, fish, along with such less familiar types as lampreys and hagfish (Cyclosto mata, q.v.). The name is not precisely equivalent to Chordata: the latter name is used to include in addition to typical verte brates, the Tunicata (q.v.), which are universally accepted as degenerate relations of the vertebrates, and also certain other types such as Balanoglossus (q.v.) and Pterobranchia (q.v.) whose genetic affinity with the Vertebrata is more doubtful. The phylum is marked off from all others by a plan of bodily structure peculiar to itself, including (I) an axial supporting skeleton traversing the body longitudinally in the mesial plane, (2) a muscular system consisting primarily of longitudinal muscle fibres situated to right and left of the axial skeleton, and (3) the concentration of the central nervous system and the main blood vessels in longitudinal trunks in the region of the mesial plane, the nervous system dorsal to, and the great vessels, as well as the other main organs of the body, ventral to the axial skeleton.
The axial skeleton in its primitive condition, as seen in one of the lower types or as a temporary phase in the embryos of the higher, consists of a stiff rod, the notochord, cellular in nature, its stiffness due to the distension of its constituent cells by fluid secreted in their interior. In the more typical vertebrates this continuous notochord gives place to a jointed chain of rigid vertebrae, giving increased flexibility combined with more effi cient support.
The muscular system shows the peculiarity that the longitud inal fibres composing it are limited in length to that of a single mesoderm segment, so that the system consists of a series of paired blocks or myotomes, each composed of a mass of longi tudinal fibres. The physiological significance of this arrangement is that contraction of the myotomes in turn from the head end backwards produces waves of lateral flexure which, driven back along the body and acting against the resistance of the external medium, bring about forward movement of the body as a whole.
The construction of the body for such eel-like movement is per haps the most fundamental feature of vertebrates and it is in accordance with it that the important longitudinal conducting organs of the body, such as central nervous system and main blood vessels, whose functions would be seriously interfered with by compression, are situated mesially.
The adaptation of the vertebrate to forward movement in a definite direction carries with it correlated modifications in struc ture of the terminal portions of the body. In front, special paired sense-organs are developed for the reception of impressions from the outer world—chemical (olfactory organs), or optical (eyes; peculiar in that they are myelonic, i.e., developed out of the side of the tubular nerve cord) or mechanical (otocyst). There fol lows in the neighbourhood of these sense-organs a concentration of the special nerve-centres, accommodated by expansion of the central nervous system to form the brain. The mouth too is situated near the anterior end, and the alimentary canal (pharynx) immediately behind the buccal cavity shows characteristic per foration of its side-walls by a series of visceral clefts whose vascular walls form respiratory organs (gills). In compensation for the resulting weakening of the pharyngeal wall, the mass of tissue between adjacent clefts ("visceral arch") develops in its interior a skeletal hoop of cartilage or bone. These skeletal arches become modified in detail in various ways and, in the case of the anterior one, these modifications form the jaws that sup port the margins of the mouth-opening. The mouth-opening of the primitive vertebrate appears to have been situated on the ventral side of the head under a forwardly projecting, over hanging lobe, a position which it still retains in the shark-like fishes to-day. The anal opening similarly was possibly situated close to the hinder end of the body, but there is a characteristic tendency for it to become displaced forwards along the ventral side of the body, reaching its maximum in some of the teleostean fishes, where the anus is jugular.