Branch 3.—Arthropoda (Gnathopoda). Class.—Arachnid a.
Arthropoda developed from ancestral forms, in which a ‘prosoma ‘formed by the union of the prostomium and six anterior segments was sharply marked off from the rest of the body, both by the confluence of its terga to form a carapace and by the special character and size of its appendages. The six pairs of appendages (including the foremost of the whole series) were arranged round the mouth, and all subservient to the purpose of prehension and mastication of food. In the later developed forms of Arachnida either the number of these appendages may be reduced (Euryp- terina, Trilobita), or the functional relation to the mouth of the more posterior of the six pairs may be lost. Whatever their number, the foremost pair is free from a jaw-like enlargement of the coxa. The palps of all six pairs of appendages exhibit a wide range of adaptational form, as prehensile, tactile, ambulatory, natatory, or fossorial organs.
The generative apertures are placed far forward—ancestrally in the first segment of the ‘mesosoma’ or region following the prosoma, and are covered by a fused pair of appendages, or, when these have aborted, by the corresponding sternite.
The appendages of the mesosoma posterior to the generative apertures carry peculiar respiratory lamellae, which expose the blood circulating in them to the dissolved oxygen of natural waters in the more archaic members of the group, but are perforated, invaginated in recesses of the ventral integument, and filled with atmospheric air in terrestrial forms (Scorpions, Spiders, &c.), or may be altogether aborted and replaced by tracheae.
Except in theTrilobita the segments and paired appendages of the mesosoma are not more than six in number, and the same is true of the metasoma or terminal region of the body, which is devoid of appendages (except in Trilobita), and may either have the appearance of a simple continuation of the mesosoma (macrourous forms), or may have its segments fused with one another, but separate from those of the mesosoma (Trilobita) ; or, again, may be more or less completely aborted and fused with the mesosoma (Limulus), when the segmentation of the mesosoma itself may also become partially (Spiders) or completely (Acarina) oblite- erated.
In all the larger known forms (Limulus, Scorpio, Mygale) a large free sclerite, the entosternite, is found within the prosoma, giving attachment to muscles inserted into the sternites of the mesosoma.
Tabular view of the Orders of Arachnida.