The five sub-kingdoms which we have previously considered - viz., the Protozoa, Coelenterata, Echinodermata, Annulosa, and Mollusca - were grouped together by the French naturalist Lamarck to form one great division, which he termed Invertebrata, the remaining members of the animal kingdom constituting the division Vertebrata. The division Vertebrata, though including only a single sub-kingdom, is so compact and well marked a division. and its distinctive characters are so numerous and so important, that this mode of looking at the animal kingdom is, at any rate, a very convenient one.
The sub-kingdom Vertebrata may be shortly defined as comprising animals in which the body is 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 haemal regions of the body are always completely shut off from one another by a partition; the limbs are never more than four in number, and are always turned away from the neural aspect of the body ; mostly there is the bony axis known as the "spine" or "vertebral column" and in all the structure known as the "noto-chord" is present - in the embryo, at any rate. These characters distinguish the Vertebrata, as a whole, from the Invertebrata; but it is necessary to define these broad differences more minutely, and to consider others which are of little less importance.
One of the most obvious, as it is one of the most fundamental, of the distinctive characters of Vertebrates, is to be found in the shutting off of the main masses of the nervous system from the general cavity of the body. In all Invertebrate animals, without exception, the body (fig. 238, A) may be regarded as a single tube, enclosing all the viscera; and consequently, in this case, the nervous system is contained within the general cavity of the body, and is not in any way shut off from the alimentary canal. The transverse section, however, of a Vertebrate animal exhibits two tubes (fig. 238, B), one of which contains the great masses of the nervous system - that is, the cerebro-spinal axis, or brain and spinal cord - whilst the other contains the alimentary canal and the chief circulatory organs, together with certain portions of the nervous system known as the "ganglionic" or "sympathetic" system. Leaving the cerebro - spinal centres out of sight for a moment, we see that the larger or visceral tube of a Vertebrate animal contains the digestive canal, the haemal system, and a gangliated nervous system. Now this is exactly what is contained in the visceral cavity of any of the higher Invertebrate animals; and the opinion has been generally entertained that it is the sympathetic nervous system of Vertebrates which is truly comparable to, and homologous with, the nervous system of Invertebrates. On the other hand, there are Invertebrates with a sympathetic system of nerves, and the development of the nerve-chain of the Annu-losa resembles that of the cerebro-spinal axis of the Vertebrata. The tube containing the cerebro-spinal centres is formed as follows: At an early period in the development of the embryo of any Vertebrate animal, the portion of the ovum in which development is going on - the "germinal area" - becomes elevated into two parallel ridges, one on each side of the middle line, enclosing between them a long groove, which is known as the " primitive groove" (fig. 239, A, B). The ridges which bound the primitive groove are known as the " laminae dorsales;" and they become more and more raised up, till they ultimately meet in the middle line, and unite to form a tube, within which the cerebro-spinal nervous centres are developed. It follows from its mode of formation that the inner wall of the tube formed by the primitive groove, which remains as the septum between the cerebro-spinal canal and the body-cavity, is nothing more than a portion of the primitive wall of the body of the embryo. And there appears to be little doubt, as believed by Remak and Huxley, that the cerebro-spinal nervous centres are "the result of a modification of that serous layer of the germ, which is continuous elsewhere with the epidermis " (Huxley).
Fig. 238. - A, Transverse section of the body of one of the higher Invertebrata: a Body-wall; b Alimentary canal; c Haemal system; n Nervous system. B, Transverse section of the body of a Vertebrate animal: a Body-wall; b Alimentary canal; c Haemal system ; n Sympathetic system of nerves; n Cerebro-spinal system of nerves; ch Notochord.
Another remarkable peculiarity as regards the nervous system is found in the fact that in no Vertebrate animal does the alimentary canal pierce the main masses of the nervous system, but turns away to open on the opposite side of the body. In most Invertebrates, on the other hand, in which there is a well-developed nervous system, this is perforated by the gullet, so that an oesophageal nerve-collar is formed, and some of the nervous centres become prae - oesophageal, whilst others are post-oesophageal.