ARTICULA'TA, or ARTICULATED ANIMALS, one of the great primary divisions of the animal kingdom, according to the system of Cuvier, who in this is followed by recent naturalists generally. The term indicates not the possession of articulated members, but the articulated structure of the whole body. The A. are composed of segments articu lated or jointed together in a line, each segment being formed of one or more rings, which in sonic appear externally as mere transverse folds in a soft skin, but are often covered with a hard substance similar in chemical composition to the bones of vertebrated animals. To this the muscles are attached, and it has sometimes received the name of an external skeleton—a name perhaps suggestive of closer and more numerous analogies to the bony framework of the vertebrated animals than actually exist. In some of the A. the rings are almost equally developed; in others, the differ ence is very great. They are divided into those which have and those which have not articulated members, the first subdivision including insects, arachnids, crustacea, and myriapoda; the latter, annelida and entozoa. Sonic naturalists rank cirrhopoda (barnacles, acorn-shells, etc.) among the A., and regard them as intermediate between these two subdivisions; others follow Cuvier in placing theta among the mollusca. The rotifera (or wheel animalcules) are also placed by sonic in the second subdivision of the A., but their right to be so placed is by no means well established. It is in the first subdivision only that the rings are very distinctly grouped in what are called segments of the animal; and even in the myriapoda (centipedes, jai, etc.) they often seem little else than mere repetitions of each other; whilst in some of the crustacea, as the crabs, the trunk becoming encased in a hard envelope, the segments become immovably united, so that they no longer appear as distinct. A few only of the lowest A., however, are destitute of a distinct head, in which are placed the eyes and other organs of special senses, with regard to which there is considerable difference in the different classes. In it also they usually have jaws for seizing their food and cutting or tearing it to pieces. Their jaws do not open vertically, as in vertebrate animals, but laterally; and there are frequently several pairs of them. Some, however, have the mouth adapted merely for suction. The alimentary tube often proceeds in a straight line from one extremity of the body to the other; and when it is convoluted, its convolutions are usually few. There is no proper heart; but instead of it. we find a dorsal yowl. a tube carried along the
central line of the body near the back or upper side, and divided in a manner corre sponding with the division of the body into rings and segments; a general connection being thus maintained, whilst each segment or each ring has to a certain extent a system of circulation for itself. Respiration is effected either by gills (bronchia), which is the case in those A. that live in water, or by air-tubes (trachea) and sacs; and the oration of the blood taking place not merely in one or two, but in many of the rings, great mus cular power and activity are maintained without a very active circulation. The nuNcular power is, indeed, greater in proportion to the size in the A. than in any other animals. The blood is usually white; in some of the annelida alone it is red; but this color tsee ANNELIDA) does not indicate any approach to the higher classes of animals, although even Cuvier appeals to have regarded it as a reason for assigning to the annelida the first place among the A. The nervous system exhibits a great similarity throughout the whole of the A., and corresponds in its general plan with their system of circulation. It consists of a series of small nervous masses or ganglia, arranged in a chain along the central line of the body on the under side of the animal. A ganglion in the bead is often termed the brain, and from it proceed the optic nerves and other nerves of the special senses; but it by no means -perfeetly correspond,to in vertebrate ganglia themselves are animals. There is usually a ganglion for each ring. The double or composed of two halves, more or less distinctly separ ated; the connecting cord also is double. In those A. which have articulated limbs, the ganglia arc largest in the parts of the trunk with which the limbs are connected, whilst they almost disappear from the more unimportant rings; in the crabs, and some other tailless or very short tailed crustacea, they are condensed into two masses.
The remains of the A. in the fossiliferous rocks are numerous, although often so fragmentary and imperfect that the determi nation of genus and species is impossible, and their complex organization cannot be thoroughly investigated. It is evident, however, that many of them differed much from any animals now known to exist, and changes can be observed from one geologic period to another; the great crustacean family of the trilobites (q.v.), for example, being found only in the paheozoic rocks. Markings, supposed to be the tracks and burrows of marine worms, appear among the earliest traces of animal life.