VERTEBRATE ANIMALS (VERTEBRATA, ante). The following is a brief synop tical classification of the principal divisions of the vertebrate branch of the animal kingdom, including, with few exceptions, only classes and orders, and referring to various articles throughout this work, for a description of families, genera, and species. The vertebrate animals include only one 'subkingdom, vertebrata, which may be briefly defined as comprising animals having a body composed of a number of segments placed on a longitudinal axis, and a nervous system chiefly dorsal. A transverse and vertical section of the body exhibits two cavities completely separated from each other 6y a partition, The dorsal, or cerebro-spinal nervous m us syste , is contained in the superior or dorsal cavity; the ventral, or inferior cavity, contains the alimentary canal, heart, and home] system, and also a nervous system, called the sympathetic, or pn„ffilonic, consisting usually of a double chain of ganglia, connected by nerve nbert, which also contain filaments derived from the cerebro-spinal system. Du ring the embry mic condition of all vertebrates the center of the partition is occupied an i elon sated, cylindroidal mass, called the notochord, or chorda dorsalis, which in i ,some vertebrate continues through life; but it is usually replaced by a partly fibrous. and cartilaginous, and partly bony, jointed structure, called the vertebral column. See DEVELOPMENT OF Tiur. EMBRYO, ante. Another characteristic of vertebrates is that the alimentary canal never penetrates or passes any part of the nervous system as it does in invertebrates, the gullet in them passing through an cesopliagal nerve collar. In all vertebrates, moreover, that part of the wall of the visceral tube or cav ity, which lies at the sides of, and immediately behind, the mouth, has, at a certain stake of development, a series of parallel thickenings, transverse to the axis of the body, which are called the visceral arches, and between these arches there are clefts by which the pharyngeal cavity temporarily communicates with de exterior. Nothing corre sponding to these arches and clefts is found in the invertebrate. A vertebrate may have no articulated limbs, and it never has more than two pairs, which always have an internal skeleton to which the muscles are attached. Invertebrates usually have a greater number of limbs, and the muscles are attached to an exo-skeleton. All vertebrates, except the lancelet, have a single valvular heart, and a hepatic portal sys tem, the blood from the alimentary canal being in part returned to the heart by a portal vein which ramifies through the liver; and there is also a peculiar system of vessles, called the lacteal system, which is an appendage to the venous system of blood vessels, and consists of a series of vessels which take up the products of digestion, and while they are undergoing partial elaboration, convey them to an entrance into the venous circulation, where they mingle with the venous blood, soon to be sent to the lJings. In regard to the cerebro-spinal and sympathetic nervous systems, the
former constitutes by far the greater portion, and in all except the lowest members. there is a well-marked separation into spinal cord (myelon) and brain (encephalon), the proportion of one to the other differing very greatly. The nerves which arise from the cerebro-spinal axis are symmetrically disposed on the two sides of the body, and are chiefly concerned in sensation and motion, and the cerebro-spinal centers also are symmetrically formed, while the sympathetic system is more or less unsymmetrical in form and distribution, and is chiefly concerned in regulating the functions of diges tion, circulation of the blood, and -lymph, and, to an undetermined extent, respira tion.
The cerebro-spinal system is commonly called the system of animal life, while the system is called that of organic or vegetattve life, as it has nothing to do, so far as can be ascertained, with voluntary motion. It may be remarked here, how ever, that the physiology of the nervous system, in some respects, is far from being perfectly understood, although almost innumerable important facts have been ascertained. Although the articulate, such as insects and crustaceans, are particularly associated with the possession of an exo-skeleton exclusive of an endoskeleton, such as belongs. to vertebrates, still many of the latter have an exo-skeleton, formed by a hardening of the outer layer of the integument. The integument is composed of two layers, an outer non-vascular epidermis, and a deeper.vascular dermis, and from these may be developed an epidermal, or a dermal skeleton. The epidermal skeleton is always horny, and is represented under various forms, as in the hairs of animals, the feathers of birds. the scales of serpents and lizards, and the plates of tortoises and turtles. The dermal skeleton may be either horny or bony.
Divisions of the Vertebrata. —This great subkingdom is naturally divided into the five• great classes of fishes, amphibians, reptiles, birds, and mammals, or pisees, amphilda, aces, and mammalm. As has been mentioned in the preceding article, however, there are differences of opinion in regard to the relations of members of these classes, so that there is disagreement when it is undertaken to group them into larger sections. The fact that all agree in recognizing the above five divisions, while many high authorities disagree hi regard to more general groupings, suggests a question as to the necessity of a more general division.