The three systems now enumerated—the digestive, the nervous, and the muscular—are readily demonstrated in the majority of the symmetrical animals, and are even very soon found to have acquired complication, and to have sundry other parts and organs superadded to them. The digestive apparatus consists of a mouth for the suseeption of aliment, of a stomach for its elaboration, of an intestinal canal from which the nutrient juices are ab sorbed, and of an anus from which the un digested residue is expelled. Whilst in the radiata the nutritious fluids passed through the parietes of the digestive cavity to impregnate the body of the animal, and be assimilated with its substance ; in the binaria we find vessels, the rudiments of a circulating system, employed in receiving the juices prepared in the digestive apparatus and transmitting these to all parts of the body. Digestion, too, in this class becomes a more complicated process than in the radiata, and various secreted fluids, saliva and particularly bile, the special products of large and evidently important organs, are added to the alimentary mass in its progress through the intestinal canal.
In addition to the digestive apparatus and general external respiratory surface we by-and by find an especial system dedicated to the aeration of the juices prepared for nutrition ; this is the respiratory apparatus. Of extreme simplicity in the first instance, being little or no more than a fold of integument turned inwards, and forming a simple cavity or sac within the body of the animal, it is soon rendered more complex in its structure, being distributed in the manner of vessels under the name of trachea or canals to different parts of the body, or being confined to a particular district, and entitled lungs or gills as it is fitted to receive the atmospheric air immediately, or to make use of this elastic fluid suspended or dissolved in water.
The existence of this separate respiratory apparatus presupposes that of another system, namely, the circulatory. The fluids prepared by the organs of digestion are not yet fitted to minister to the growth and nutrition of the organization ; to be made apt for this purpose they require exposure to the air in the lungs or gills wherever these organs exist, and these be ing distinct, or contained in a particular region of the body, a series of conduits were re quired, first to carry the fluids thither, and to transmit them subsequently to every part of the organization for its support. Like all the other systems of animals, the circulatory exists of various degrees of complexness; when first encountered it consists of a series of simple canals or vessels, which diverge on every band ; by-and-by it has several, and finally one, forc ing piece, or heart superadded to it, which impels the fluids by its contractions to every the most remote part of the organization.
Among animals, however, nutrition is not a process simply of addition or composition ; it is also, perhaps universally, one of subtrac tion or of decomposition. We have seen the composition provided for by special systems in animals occupying very low grades in the scale of creation ; we mount but a short way before over encounter an apparatus which pre sides over the decomposition also in the shape of another system of vessels, the veins and especially the lymphatics ; these collect the superfluous and worn-out particles from every part, pour them into the general current of the circulation, wherein being exposed in the vital elaboratory of the lungs they are either assi milated anew and made fit once more to form an integral part of the organization, or, being subjected to the action of certain glands, they are singled out, abstracted, and finally ejected from the system entirely. In the most com plicated animals therefore a peculiar appa ratus for the depuration of the system is su peradded as complementary to the absorbents. This we find in the glandular bodies familiarly known as the kidneys ; the vehicle in which the decayed particles are withdrawn is the urine.
When we examine the instruments of sensa tion, we find them becoming gradually more and more numerous, and the nervous system generally more and more complicated as we rise in the scale of animal creation. The ner vous system is before long found to consist of other parts than a series of similar ganglions supplying at once the organs of sensation and those of digestion ; it has a central part super added, from which issue immediately the nerves that supply the organs of the senses,— sight, hearing, taste, and smell, which at the same time make their appearance with their especial capacities. This central superadded portion is the brain, with its prolongation in the vertebrata entitled spinal marrow. Nor in the more perfect classes of the animal king dom is the nervous system even thus simple ; among them it consists essentially of two grand divisions, the one including the brain and spinal cord and the nerves thence pro ceeding, the other constituted by the system of the great sympathetic, or that series of ganglions which, situated on either side of the vertebral column, from the head to the pelvis, are con nected with one another, and with the cerebro spinal system, by branches of communication, and furnish the digestive apparatus with almost the whole of the numerous nerves it receives.