VERTEBRATE ANIMALS.

CHAPTER LI.

GENERAL CHARACTERS AND DIVISIONS OF THE
VERTEBRATA.

THE five sub-kingdoms which we have previously considered— viz., the Protozoa, Calenterata, Annuloida, 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 hæmal 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 "notochord" is present-in the embryo, at any rate. These characters distinguish the Vertebrata, as a whole, from the Inverte brata; 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 Inverte brate animals, without exception, the body (fig. 221, A) may be regarded as a single tube, enclosing all the viscera; and

Fig. 221.-A, Transverse section of the body of one of the higher Invertebrata: a Body-wall; 6 Alimentary canal; c Hæmal system; " Nervous system. B, Transverse section of the body of a Vertebrate animal; a Body-wall; & Alimentary canal; c Hamal system; n Sympathetic system of nerves; ' Cerebro-spinal system of nerves; ch Notochord.

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. 221, 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 hæmal 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 it follows from this, as pointed out by Von Baer, that it is the sympathetic nervous system of Vertebrates which is truly comparable to, and homologous with, the nervous system of Invertebrates. The cerebro-spinal nervous centres of the Vertebrata are to be regarded as something superadded, and not represented at all amongst the Invertebrata.

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. 222, A, B). The ridges. which bound the primitive groove are known as the "laminæ 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).

Another remarkable peculiarity as regards the nervous system is found in the fact that in no Vertebrate animal does the

Fig. 222.-Embryology of Vertebrata. A, Portion of the germinal area of the ovum of a Bitch, showing the primitive groove (after Bischoff). B, Profile view of the same. C, Diagram representing the amnion and allantois: e Embryo; a Amnion; a Umbilical vesicle; b Allantois; Pedicle of the allantois, afterwards the urinary bladder. D, Head of an embryo, showing the visceral arches (vv).

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 œsophageal nerve collar is formed, and some of the

nervous centres become præ-œsophageal, whilst others are post-œsophageal.

Furthermore, the floor of the "primitive groove" in the embryo of all Vertebrates has developed in it at an early period the structure known as the "notochord" or "chorda dorsalis" (fig. 221, B, ch). This structure, doubtfully represented in any Invertebrate, is a semi-gelatinous or cartilaginous collection of cells, forming a rod-like axis, which tapers at both ends, and extends along the floor of the cerebro-spinal canal, supporting the cerebro-spinal nervous centres. In some Vertebrates, such as the Lancelet (Amphioxus), the notochord is persistent throughout life. In the majority of cases, however, the notochord is replaced before maturity by the structure known as the "vertebral column or "backbone," from which the sub-kingdom Vertebrata originally derived its name. This is not the place for an anatomical description of the spinal column, and it is sufficient to state here that it is essentially composed of a series of cartilaginous, or more or less completely ossified, segments or vertebra, arranged so as to form a longitudinal axis, which protects the great masses of the nervous system. It is to be remembered, however, that all Vertebrate animals do not possess a vertebral column. They all possess a notochord; but this may be persistent, and in many cases the development of the spinal column is extremely imperfect.

Another embryonic structure which is characteristic of all Vertebrates, is found in the so-called "visceral arches" and "clefts" (fig. 222, D). The "visceral arches" are a series of parallel ridges running transversely to the axis of the body, situated at the sides of, and posterior to, the mouth. As development proceeds, the intervals between these ridges become grooved by depressions which gradually deepen, until they become converted into a series of openings or "clefts," whereby a free communication is established between the upper part of the alimentary canal (pharynx) and the external medium. In Fishes and many Amphibians the greater number of the visceral clefts remain open throughout life; and the visceral arches of all fishes (except the Lancelet) throw out filamentous or lamellar processes, which receive branches of the aorta and constitute branchiæ. In the higher Vertebrata all the visceral clefts become closed, whilst no branchia are ever developed upon the visceral arches.

The limbs of Vertebrate animals are always articulated to the body, and they are always turned away from the neural aspect of the body. They may be altogether wanting, or they

may be partially undeveloped; but there are never more than two pairs, and they always have an internal skeleton for the attachment of the muscles of the limb.

A specialised blood-vascular or "hæmal" system is present in all the Vertebrata; and in all except one-the Amphioxusthere is a contractile cavity or heart, which never consists of less than two chambers provided with valvular apertures.

the Vertebrata the heart is essentially a respiratory heartthat is to say, it is concerned with driving the impure or venous blood to the breathing organs; and in its simplest forms (fishes) it is nothing more than this. In the higher Vertebrates, however, there is superadded to this a pair of cavities which are concerned in driving the pure or arterial blood to the body. In the case of the Mammals, these two circulations are often spoken of as the "lesser" or "pulmonary" circulation, and the "greater" or "systemic" circulation.

In all Vertebrates there is that peculiar modification of the venous system which is known as the "hepatic portal system." That is to say, a portion of the blood which is sent to the alimentary canal, instead of returning to the heart by the ordinary veins, is carried to the liver by a special vessel-the vena porta-which ramifies through this organ after the manner of an artery.

In all Vertebrates, also, is found the peculiar system of vessels known as the "lacteal system." This is to be regarded as an appendage of the venous system of blood vessels, and consists of a series of vessels which take up the products of digestion from the alimentary canal, elaborate them, and finally empty their contents into the veins.

Lastly, the masticatory organs of Vertebrates are modified portions of the walls of the head, and never "hard productions of the alimentary mucous membrane, or modified limbs" (Huxley), as they are amongst the Invertebrata.

The above are the leading characters of the Vertebrata as a whole; but before going on to consider the primary divisions of the sub-kingdom, it may be as well to give a very brief and general description of the anatomy of the higher and more typical Vertebrates, commencing with their bony framework, or skeleton.

The skeleton of the Vertebrata may be regarded as consisting essentially of the bones which go to form the head and trunk on the one hand (sometimes called the "axial" skeleton), and of those which form the supports for the limbs ("appendicular " skeleton) on the other hand. The bones of the head and trunk may be looked upon as essentially composed of a series