G. Arthropoda (Jointed-limbed Animals). By Fred. V. Theobald, M.A.

Bilaterally symmetrical segmented animals which are covered with a thick cuticle that is frequently calcareous (Crustacea), but always thinner between the segments; they carry (primitively) a pair of jointed appendages on every segment. The segments of the body are uniform in certain regions, but differ from those of contiguous regions, so that it is easy to distinguish three parts (head, thorax and abdomen), each composed of segments. The cephalic segments are always formed into a uniform head, the segmentation being scarcely recognizable at either end; the thoracic segments may also fuse, or part or all of them may coalesce with the head; the abdomen, as a rule, retains its segmentation, but this may possibly also be lost, in which case it is [sometimes] united to the cephalothorax. The structure of the three regions depends mostly on the varying form and function of the appendages: those on the head are primitively locomotive organs (and frequently are still so in the early stages), but they become transformed into feelers and mouth-parts (mandibles, maxillæ); the limbs of the thorax, however, usually retain their ambulatory functions, as frequently do those of the abdomen; sometimes, however, the abdominal limbs disappear, entirely or partly; in the latter case they are then utilized for other purposes.

In their organization the Arthropoda approach the segmented worms.

The Arthropoda are generally divided into five groups (Crustacea, Protracheata, Arachnoidea, Myriapoda, and Insecta or Hexapoda), of which only the Arachnoidea and the Hexapoda interest us here.