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...
