Articulata, the third great division of the animal kingdom in the classification of Cuvier, and by him subdivided into four classes. Other naturalists have added four more, making the following eight classes, of which the first four are those of Cuvier:
1. Annelida, as leeches, earthworms. etc.
2. Crustacea, as crabs, lobsters, prawn, shrimps, etc.
3. Arachnida, as spiders, scorpions, mites. etc.
4. Insecta, as beetles, flies, butterflies, etc.
5. Myriopoda, as centipedes.
C. Cirrhopoda, as barnacles and sea acorns.
7. Rotifcra. wheel-shaped animalcules, aquatic.
8. Entozoa - lowest of the worms - parasites upon or within other animals.
Each of these classes will be found treated under its own name. The articulata may properly be ranked, upon the whole, as higher in the animal scale than the mollusca, although, as in this division, some species may be found less highly organized than are some of the radiata, the fourth division of the series; for the articulata possess a high development of the locomotive organs, in which the mollusca are particularly deficient. The nervous system also is so organized that it presents a sufficient characteristic for designating the group; and the name homogangliata has been proposed by Prof. Owen as a substitute for that of articulata, this having reference only to the external conformation of the body in transverse rings, which may be of the soft skin or integument, or else serve, in the form of a hard shell, as an external skeleton, to which the muscles are attached. This arrangement of the nerves is a chain of knots or ganglia, symmetrically disposed upon a double cord, which passes through the ventral region of the body, and from each ganglion nervous filaments pass off to the different segments of the body. A nervous ring from the anterior pair of ganglia encircles the oesophagus.
Filaments connect this with the organs of the senses, and the oesophageal ganglia have hence been regarded as analogous to the brain in the higher orders. They are more and more concentrated as the animal occupies a more elevated position in the division, the members of the body being at the same time brought into closer connection. The symmetrical arrangement of the nerves suggests that of the members also; and the limbs are found arranged in pairs, in the centipedes each pair proceeding from one of the articulations of the body. In the higher classes, as the Crustacea, the same symmetry of pairs of limbs is perceived, and the connection of each pair with a segment of the body, even when the thorax, or body, needing no flexibility for locomotive purposes, has its rings very obscurely defined; The lower groups contain the greater number of articulations or rings, and these are usually soft, upon an elongated body, furnished in most cases with no true limbs. Progressive motion is obtained by the bending of the flexible body in one and another direction, the muscles which effect this occupying a large portion of the body which, in other animals is usually devoted more to the organs of nutrition and digestion. These in the articulate are not so elaborate as in the mollusca.
The organs for respiration are much more highly organized, particularly in the insecta. In the air-breathing species the blood is aerated by being exposed to the action of the air introduced within the body, the fluid being distributed in cavities or tubes permeable to the air; the former appear to be analogous to lungs. In the articu-lata is found the greatest diversity of forms and habits of life. The largest animals of the division are the lobsters and crabs of the Crustacea; the rest are, for the most part, of small size, many of them so minute as to pass unnoticed in the watery elements in which they abound.