ANIMAL KINGDOM, an appellation given to that great division of natural bodies to which ANIMALS belong. Like the other kingdoms of nature, the mineral and the vege table, it is divided into numerous sub-king doms, classes, orders, genera, and other subor dinate groups, according to the properties and forms of the objects which it comprehends. As the primary grand divisions of the mineral kingdom are founded on the primitive forms of crystallization, and those of the vegetable king dom on the endogenous and exogenous modes of growth, zoologists have endeavoured to find some common principle for their first divisions of the animal kingdom. The most common function in animals, and in all organized beings, is generation, and we find the animal kingdom divided into four distinct groups by the modifi cations of this function, viz., fissipara, gemmi para, ovipara, and vivipara. But as the .fissi parous and geminiparous modes of generation are effected without the presence of distinct permanent organs, as the fissiparous mode occurs in isolated species belonging to classes remote from each other in the scale, and as nearly all the classes of the animal kingdom belong to the oviparous division, the modifica tions of this system do not present the means of establishing primary divisions suitable for the purposes of zoology. Although the pro cess of internal digestion is not so universal as the ,Junction of generation, the internal alimentary cavity is the most universal organ of animals, and its forms therefore merit a first consideration in the establishment of pri mary groups. It is found, however,' that in animals whose general structure is nearly the same, the alimentary apparatus varies so much according to the nature of the food, as to render hopeless any attempt to subdivide the animal kingdom from its modifications ; as from its having one or two apertures, from its being a simple sac or a lengthened intestine, from its having one, two, or more stomachs or glands developed in its course, or other modifications of this kind.
In the circulating system we are presented with better means for such divisions than in the digestive, for the radiated classes have only vessels for their circulation, the articulated classes have a superadded ventricle, the mollus cous classes and fishes a bilocular heart, am phibia and reptiles a trilocular heart, and the birds and mammalia have four cavities in that organ. The respiratory organs likewise afford the means of founding primary divisions, as into ciliated, branchiated, and pulmonated classes, in ascending from the lowest to the highest forms of that system.
The primary divisions of the animal kingdom adopted by Aristotle, viz., animals with red blood and animals without red blood, are ob viously founded on a single principle of classi fication, and correspond with the more recent divisions of vertebrate and invertebrata ; but from the number of distinct classes of animals now comprehended under each of these divisions, they are quite unsuitable as primary groups in the present advanced state of the science of zoology. Considering the functions of the
nervous system or the intellectual conditions of animals as a 'means of classification, Lamarck proposed three ()Teat divisions, the lowest of which comprehended the animals regarded by him as apatliic or automatic, the second the sensitive, and the highest the intelligent, which, however, are too hypothetical to answer the purposes of the zoologist. Without any fixed principle for the establishment of his primary groups, Cuvier divided the animal kingdom into the radiated, the articulated, the mo//uscous, and the vertebrated divisions, which have been generally adopted by naturalists. From the importance of the nervous system in the living economy of animals, some have sought in its modifications a means of establishing primary or grand divisions of the animal kingdom on principles more uniform and philosophical than those commonly employed. In the radiated or lowest classes of animals, wherever the nervous system is perceptible, as in actinic, medusa, beroe, asterias, echinus, holothuria, &c. it is found in the form of filaments disposed in a circular manner around the oral extremity of the body. This lowest form of the nervous system is expressed by the term cyclo-neura, and although, like the radiated and every other character assigned 'to these classes, it is of partial application, it marks the uniform con dition of that system on which the manifesta tions of life are chiefly dependent, and which principally establishes the relations of animals to surrounding nature. A different form of the nervous system is found in the long cylindrical trunks of the helminthoid and entomoid classes, where we observe almost from the lowest ento zoa to the highest crustacea, a double nervous chord or column extending along the whole of the ventral surface of the body. This form of the nervous system, common to the articulated classes of animals, is expressed by the term diplo-neura, and it is found to accompany an organization generally more complex than that of the cyclo-neurose classes, and inferior to that of most of the succeeding divisions or sub kingdoms, especially in the organs of vegetative or organic life, as the vascular, the digestive, and the glandular apparatus. The nervous system is more concentrated around the en trance to the alimentary canal in the mollus cous classes, where it generally forms a trans verse series of ganglia, disposed around the oesophagus, a character which is expressed by the term The dorsal position of the great ganglia and nervous columns of the cephalopods, and their partial protection by an organised osseous internal skeleton, leads to the condition of the nervous system presented by the vertebrated classes of animals, where its central parts are in the form of a lengthened dorsal nervous chord developed anteriorly into a brain, and protected by a vertebral column and nium. The vertebrated classes are thus de signated from the form of the most influential part of their organization.