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  Section: Anatomy of Vertebrate Animals » Organisation of the Vertebrata Skeleton
 
 
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The Distinctive Characters of the Vertebrata

 
     
 

The Vertebrata are distinguished from all other animals by the circumetance that a transverse and vertical section of the body exhibits two cavities, completely separated from one another by a partition. The dorsal cavity contains the cerebro-spinal nervous system; the ventral, the alimentary canal, the heart, and, usually, a double chain of ganglia, which passes under the name of the "sympathetic." It is probable that this sympathetic nervous system represents, wholly or partially, the principal nervous system of the Annulosa and Mollusca. And, in any case, the central parts of the cerebro-spinal nervous system, viz., the brain and the spinal cord, would appear to be unrepresented among invertebrated animals. For these structures are the results of the metamorphosis of a part of the primitive epidermic covering of the germ, and only acquire their ultimate position, in the interior of the dorsal tube, by the development and union of outgrowths of the blastoderm, which are not formed in the Invertebrata. (It is possible that an exception to this rule may be found in the Ascid- ans. The tails of the larva of these animals exhibit an axial structure, which has a certain resemblance to a vertebrate notochord; and the walla of the pharynx arc perforated, much as in Amphioxus.)

Again, in the partition between the cerebro-spinal and visceral tubes, certain structures, wbich are not represented in iuvertebrated animals, are contained. During the embryonic condition of all vertebrates, the centre of the partition is occupied by an elongated, cellular, cylindroidal mass-the notochord, or chorda dorsalis. And this structure persists throughout life in some Vertebrata; but, in most, it is more or less completely replaced by a jointed, partly fibrous and cartilaginous, and partly bony, vertebral column.

In all Vertebrata, that part of the wall of the visceral tube which lies at the sides of, and immediately behind, the mouth, exhibits, at a certain stage of embryonic development, a series of thickenings, parallel with one another and transverse to the axis of the body, which may be five or more in number, and are termed the visceral arches. The intervals between these arches become clefts, which place the pharyngeal cavity, temporarily or permanentlj', in communication with the exterior. Nothing corresponding with these arches and clefts is known in the Invertebrata

A vertebrated animal may be devoid of articulated limbs, and it never possesses more than two pairs. These are always provided with an internal skeleton, to which the muscles moving the limbs are attached. The limbs of invertebrated animals are commonly more numerous, and their skeleton is always external.

When invertebrated animals are provided with masticatory organs, the latter are either hard productions of the alimentary mucous membrane, or are modified linabs. Vertebrated animals also commonly possess hard productions of the alimentary mucous membrane in the form of teeth; but their jaws are always parts of the walls of the parietes of the head, and have nothing to do with limbs.

All vertebrated animals have a complete vascular system. In the thorax and abdomen, in place of a single peri-visceral cavity in communication with the vascular system, and serving as a blood-sinus, there are one or more serous sacs. These invest the principal viscera, and may or may not communicate with the exterior-recalling, in the latter case, the "atrial cavities of Mollusca.

In all Vertebrata, except Amphioxus, there is a single valvular heart, and all possess an hepatic portal system; the blood of the alimentary canal never being wholly returned directly to the heart by the ordinary veins, but being more or less completely collected into a trunk-the portal vein, which ramifies through and supplies the liver.
 
     
 
 
     



     
 
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