(555). The next great division of the animal kingdom includes an immense number of living beings, adapted by their conformation to exist under a far greater variety of circumstances than any which we have hitherto had an opportunity of examining, all of which are obviously only adapted to an aquatic life, and accordingly are invariably found either to inhabit the waters around us, or to be immersed in the juices of living animals upon which they subsist. Even the Echinodermata are too imperfect in their construction to admit of their enjoying a terrestrial existence, inasmuch as, possessing no nervous centres adequate to give force and precision to their movements, they are incapable of possessing external limbs endowed with sufficient power and activity to be capable of progression upon land; neither are any of them furnished with organs of sense, that must be indispensable for the security of creatures exposed to those innumerable accidents to which the inhabitants of a rarer element are perpetually liable.

(556). The type of structure met with in the Homogangliata admits of far higher attributes and allows the enjoyment of a more extended sphere of existence: senses become developed proportionate to the increased perfection of the animal; limbs are provided endowed with strength and energy commensurate with the development of the nervous ganglia that direct and control their movements; and instincts are manifested in relation with the increased capabilities and more exalted powers of the various classes as they gradually rise above each other in the scale of animal development.

(557). The most obvious though not the most constant character that distinguishes the creatures we are now about to describe is met with in their external conformation: they are all of them composed of a succession of rings formed by the skin, or outward integument, which from its hardness constitutes a kind of external skeleton, supporting the body, and giving insertion to the muscles provided for the movements of the animal. In the lowest forms of the Articulata, the body is extremely elongated, and the rings proportionately numerous; the integument, moreover, is soft and yielding, and, as a necessary consequence, the limbs appended to the different segments are feeble and imperfect: such is the structure met with in the Worms, or Annelidans, properly so called.

(558). As we advance, we perceive the tegumentary rings to become less numerous, and the skin of a denser and firmer texture, adapted to sustain the action of stronger and more powerful muscles; the limbs likewise become more elaborately formed, their movements more free and energetic, and the instruments of sight and touch begin to assume considerable perfection of structure. This state of development we find in the Myriapoda, or Centipedes.

(559). In the Insects, the concentration of the external skeleton is still more remarkable. The integument assumes a hardness and solidity proportioned to the vigorous movements of which the limbs are now capable; the rings or segments of the body, hitherto distinct, become more or less firmly soldered together in those parts where great strength and firmness are necessary, and scarcely any traces are left to indicate their existence as separate pieces; so that, instead of exhibiting that succession of similar segments seen in the Centipede, the body is apparently divided into three distinct portions: viz. the head, that contains the organs of the senses and the parts of the mouth; the thorax, sustaining the limbs, or instruments of progression; and the abdomen, enclosing the viscera subservient to nutrition and reproduction.

(560). In a fourth division of articulated animals, namely the Arachnidans, or Spiders, still further consolidation of the external skeleton is visible; for, in them, even the separation between the head and the thorax is obliterated, and it is in the abdomen only that the segments of the body are recognizable.

(561). Lastly, among the Crustaceans we have various modifications of the outward skeleton, adapted to the habits of the different tribes. In the least perfect species, which are all aquatic, the segments of the skeleton are perfectly distinct and separate, resembling those of the Myriapoda; but in the stronger and more predacious tribes, the pieces of the head and thorax become solidly fixed together; and in those forms most adapted to a terrestrial life, namely the Crabs, almost all traces of distinction between the thoracic segments are lost in the construction of the calcareous shield that covers and protects their whole body.

(562). We see, therefore, from the above rapid sketch of the different classes composing the articulated division of the animal kingdom, that, as their organization assumes greater perfection, the different segments of the external skeleton coalesce and become united together, so as to give greater strength to those parts more immediately connected with locomotion or the destruction of prey; let us next examine the nature of the nervous apparatus that characterizes the Homogangliata, and observe the relation which the outward form of the body bears to the arrangement of this primary system of the animal economy. In the humblest forms of the Annulosa, it would seem that every ring of the body contains a complete nervous apparatus, consisting of a pair of ganglia, and a set of nerves destined to supply the particular segment in which they are lodged. All these different brains, belonging to the individual segments, communicate with each other by nervous filaments, so that a continuous chain is formed, passing along the whole length of the body.

With the exception of the anterior pair of ganglia, or those contained in the first ring, which we may call the head, the nervous centres are arranged along the ventral region of the body, that is, beneath the alimentary canal; but the anterior pair are invariably situated upon the dorsal aspect of the animal, and communicate with the rest by a nervous collar that embraces the commencement of the oesophagus. The nervous masses placed along the belly preside specially over the movements of the segments to which they belong, and have little to do with sensation, or the perception of external objects; whilst the anterior or cephalic pair, from the constancy of their communication with the organs of the senses, appear to be peculiarly in relation with the perceptive faculties of the creature.

(563). It may be taken as a general law, that the perfection of the nervous system of any animal may be estimated by the proportionate size of the central ganglia, upon the development of which both the energy of the actions of the body and the completeness of perception depend; and by following out this great principle, we shall be easily able to account for the progressive steps whereby the Articulata become more and more perfectly organized, as we trace them in the series above indicated. In proportion as we have found the segments of the body to become less numerous, the appended limbs stronger, the outward skeleton more dense, and the muscular powers more energetic, we shall find the abdominal ganglia to diminish in number by becoming consolidated into larger masses, increasing in size and energy in accordance with the development of the limbs over which they preside; and, in the same manner, we shall observe the senses assume greater perfection of structure, and the instincts become more developed, as we find the cephalic or anterior pair of brains increasing in proportionate bulk.

(564). Among the Homogangliata are likewise to be detected the first traces of the sympathetic or splanchnic nervous system. This consists of delicate filaments which are distributed upon the alimentary canal, presenting, in their course, ganglionic enlargements, and anastomosing, some with the oesophageal ring, and others with the cerebral or encephalic ganglia*.

(565). These observations will suffice to introduce the student to the Homogangliate division of the animal world, and to direct his attention to those physiological points connected with the nature of their nervous system which will be more fully laid before him in the following pages.