In the numerous classes of animals which constituted that inferior, more extensive, and diversified group, linked together by the single negative character of the absence of a vertebral column, and thence termed "Invertebrata," we saw that, as the several series became elevated in the scale of organisation, they diverged from one another by reason of the preponderating development of some particular class of organs, and culminated in species, inferior either in their general form, or their powers of motion and perception, to some of the antecedent forms, through which the series had passed. The spider and the crab are not the kinds of animals in which one should have anticipated that the type of organisation, so richly varied in the Insect class, would have ended, had that class been a step in the direct progress to the vertebrate series. The loss of wings, and the abrogation of the power of flight, would indicate a retrograde course of development. In the insect, the animal organs, more particularly those of locomotion, preponderate over the vegetative or plastic organs, and in the attempt, as it were, to restore the balance, by establishing, as in the Crustacea and Arachnida, a better defined system of circulation, and a more vigorous and concentrated heart, the general plan of the articulate structure appears not to be such as to bear this adjustment without a sacrifice of some of the faculties enjoyed by Insects. So likewise the route of organisation traceable through the molluscous type seems, on the other hand, to lead to an extreme subordination of the motive and sensitive to the vegetative systems. And in those species which make the nearest approach to the Vertebrata, we find the viscera of organic life occupying so large a proportion of the body, that no room is left for the development of nervous or muscular organs, except by what seems an undue expansion and overloading of the head, as, for example, in the Cephalopoda. In fact, the nervous system, the essence and prime distinction of the animal, had not, so to speak, any proper or defined abode in the bodies of the invertebrated animals. Its centres were sometimes dispersed irregularly through the general cavity of the body, sometimes aggregated around the gullet, sometimes arranged with more symmetry along the abdomen; yet seldom better cared for or protected than the neighbouring viscera.

The grand modification, by which a higher type of organisation is established, and one which becomes finally equal to all the contingencies, powers, and offices of animated beings, in relation to this planet, is the allocation of the mysterious albuminous electric pulp in a special cylindrical cavity, of which the firm walls rest upon a basal

*Hunterian Lectures, Invertebrata, 8vo. 1843..

axis, forming the centre of support to the whole frame, and from which all the motive powers radiate, and this axial cylinder (fig. 1. v) is called the "Vertebral Column;" vertebral, as consisting of segments of the skeleton, which turn one upon the other, and as being the centre on which the whole body can bend and rotate; from the Latin "verto, vertere," to turn.

Ideal section of a Vertebrate (Mammalian) animal.

The vertebrated animals have the nervous matter concentrated in this vertebral case, which expands at certain parts, where the largest currents of sensation enter, and those of volition go out; and more especially at the anterior or upper extremity, where the impressions to be appreciated by the nervous centre are the most varied and the most distinct. The expanded mass of nervous matter, at this part, is called the brain (fig. 1. b), the rest of the nervous axis, the spinal chord, (ch, ch); whence the highest primary group of animals is called "Myelencephala," from the Greek words signifying brain and spinal marrow. The prolongations and ramifications from these centres, forming the internuntiate channels of sensation and the will, are the nerves.

There are five special modifications of sensation in the vertebrated animals, three of which have special nerves, viz. smell (ol), sight (op), and hearing (au). Taste (t) appears to be less generally enjoyed by the Vertebrata, and its nerve is a large branch of an ordinary nerve, the fifth pair. Feeling, which, in its more exquisite degree, constitutes touch, seems a common property of all those nervous filaments, which, passing into the posterior columns of the central axis, are continued to the brain. Speaking generally, such are the attributes of the recipient or sensitive portion of the nervous axis in the Vertebrated animals. They can take cognisance of all the impressing powers which surround them; as the character and resistance of the surface which supports them, the flavour and fitness of the substances which nourish them, the purity of the atmosphere which they breathe,

the delicate vibrations of that atmosphere which follow the mutual contact or percussion of sonorous bodies, and the finer vibrations of a more subtle æther, the appreciation of which produces the sense of sight.

With these means of perceiving, knowing, and investigating the world around them, the Vertebrated animals possess a proportionate power of acting upon and subduing it. Not any species is fixed to the earth; all can move, and every variety and power of animal locomotion is manifested in the vertebrated sub-kingdom. Yet some permanently retain the worm-like figure, which all primarily manifest in common with the embryos of the articulate series; but always with the grand difference of the dorsal nervous column. Such vermiform species glide by undulatory inflections of the entire body through the waters, or on the surface of the ground. But in most Vertebrata special instruments of locomotion are developed ; some single from the median line, some in pairs; the latter never exceed four in number, two before or above, called arms, or pectoral extremities (P), and two below or behind, called legs, or pelvic extremities (V): thus, the vertebrated type is essentially tetrapodal.* The solid mechanical supporting and resisting axis, framework, or leverage (sk) of these members is internal, vascular, and commonly ossified. It is covered, and, as it were, clothed by the muscles (m), which are attached to its outer surface. The elementary contractile fibre of the voluntary muscular system is transversely striated.

The internal position of the skeleton seems to be the chief condition of the attainment, by certain Vertebrata, of a bulk far surpassing that of the largest of the Invertebrata: and the division of the skeleton into numerous pieces diversely articulated, gives great variety and precision to the movements of the Vertebrate animals.

The forms and proportions of the Vertebrata are as varied as their kinds of locomotion, and the elements in which these are exercised. With very few exceptions the body is laterally symmetrical, the right and left sides corresponding. We may likewise discern a general characteristic of the Vertebrata in the tendency to a symmetrical development, or a repetition of parts in the vertical direction; that is, in the dorsal and ventral regions. Each vertebral segment of the internal skeleton, for example, forms typically a dorsal and a ventral arch; the one protecting the nervous axis, the other the vascular trunks and organs of plastic life. The nervous trunk itself

*The homologues of these special instruments of locomotion may exist in greater numbers, more or less developed and modified, in subserviency to other functions; as, for example, the opercular and branchiostegal flaps of fishes, the simple appendages of the ribs in fishes and in birds. The arms and legs commencé in Lepidosiren, for example, as simple unbranched filamentary appendages diverging from inferior vertebral arches.

consists of dorsal and ventral columns. Whilst the Invertebrata manifest a general tendency to development in breadth, the Vertebrata rather gain in height by this doubling or repetition of parts in the vertical or dorso-ventral direction: and in this we may discern the tendency to rise above the surface of the earth, until in man the entire body is uplifted; and what is below and above in all other Vertebrata, in him becomes before and behind.

The general external integument in the Vertebrata is rarely burthened and clogged by large and massive calcareous plates, but is usually defended by light, and sometimes exquisitely organised and singularly complex developments of the epidermal covering; modified according to the spheres of existence, the habitual temperature and movements, and therefore eminently characteristic of the different classes of Vertebrated animals.

The actions of the unusually developed nervous element, - whether the vibrating filament conveys to the sentient centre impressions from without, or, obedient to the inward intelligence, imparts from within the stimulus of volition to the moving fibre,- are essentially productive of change. It is most probable that the same nervous fibre is not equally fit for two successive actions; but needs, after each, a certain amount of restoration. The same may be predicated of the action of the muscular fibre; viz., that some change, no matter how small, but to that extent unfitting it for the due repetition of the act, is the consequence of its stimulated contraction; and thus the continued existence of the living animal requires the presence of organs of renewal and repair in intimate, but harmonious combination, with those of sensation and motion.

The raw material of this restoration is derived from without: the alimentary canal, in which the conversion and animalisation of the food take place, is provided, in the Vertebrata, with two apertures, an entry or mouth (os), and an excremental outlet (as). The jaws (j) are two in number, and placed one below or behind the other, working vertically or in the axis of trunk; the principal part of the alimentary canal is contained in an abdominal cavity, and is supported by a reflection of the serous membrane upon the walls of that cavity; and the canal is divided into œsophagus (œ), stomach (g), and intestine (i). All Vertebrata have a liver (1) which is usually a very complicated gland, with a special venous or portal system of vessels; and the biliary secretion is conveyed into the commencement of the intestine. The pancreas (p), which in most Vertebrata presents the form of a compact and conglomerate gland, adds its secretion to the bile in the duodenum. The spleen (s), a cellulovascular ganglion, or gland without a duct, makes its first appearance coincidently with that of the portal vein, and manifests a progressive

development closely corresponding with that of the pancreas. Lacteal vessels convey the nutrient fluid to the veins, and thus it reaches the heart.

The central organ of circulation, always present, and of a compact muscular character, always below or anterior in position to the alimentary tube and nervous axis, is situated towards the fore-part of the body, most commonly in a compartment distinct from the abdomen, where it is suspended in a special bag or pericardium (fig. 1. h). The blood is red in all the Vertebrated animals, and the colouring matter is contained in microscopic discoid cells, of an oval or circular form (fig. 4.). The whole or part of the circulating fluid is transmitted directly from the heart to the respiratory organ (fig. 1. lg). The respiratory medium, whether air or water, is admitted to the respiratory organ by the mouth. From this organ the arterial blood is sent, sometimes directly, sometimes after a second return to the heart, or in both ways, to the rest of the system; but the breathing organs are never developed, as we saw in many of the Invertebrata, from the returning venous channels.

The venous blood in the lower Vertebrata is submitted to the depurating influence of the kidneys; but in the higher Vertebrata these de-azotising glands (k) are supplied exclusively by arteries. A part of the venous blood in all Vertebrata circulates through the liver, as through a second and subordinate lung, before it finally reaches the heart.

The system for perpetuating the species is not complete in any Vertebrated animal; that is, the generative organs are divided between two individuals, there being no natural Hermaphrodite in this sub-kingdom. Every Vertebrate embryo soon takes on its special and determinate sexual character, and ends a perfect male or perfect female -a fertiliser or a producer.

The instinctive sense of dependence upon another, manifested by the impulse to seek out a mate, which impulse, even in fishes, is sometimes so irresistible that they throw themselves on shore in the pursuit,this first step in the supercession of the lower and more general law of individual or self preservation, although not first introduced at the Vertebrate stage of the animal series, is never departed from after that stage has been gained. To this sexual relation is next added a self-sacrificing impulse of a higher kind, viz. the parental instinct. As we rise in the survey of Vertebrate phenomena, we see the entire devotion of self to offspring in the patient incubation of the bird, in the unwearied exertions of the Swift or the Hawk to obtain food for their callow brood when hatched; in the bold demonstration which the Hen, at other times so timid, will make to repel threatened attacks against her cowering young.