Class (Unica).—Onychophora.
With the characters of the grade: add the presence within the body of fine unbranched tracheal tubes, devoid of spiral thickening, opening to the exterior by numerous irregularly scattered tracheal pits.
Genera—Eoperipatus, Peripatopsis, Opisthopatus, &c. (See [Peripatus].)
Grade C (of the Arthropoda).—Euarthropoda.
(a) Integument heavily plated with firm chitinous cuticle, allowing no expansion and retraction of regions of the body nor change of dimensions, except, in some cases, a dorso-ventral bellows movement. The separation of the heavier plates of chitin by grooves of delicate cuticle results in the hinging or jointing of the body and its appendages, and the consequent flexing and extending of the jointed pieces.
(b) Claws and fangs are developed on the branches or rami of the parapodia, not on the end of the axis or corm.
(c) The head is either deuterognathous, tritognathous, or tetartognathous.
(d) Rarely only one, and usually at least two, of the somites following the mandibular somite carry appendages modified as jaws (with exceptions of a secondary origin).
(e) The rest of the somites may all carry appendages, or only a limited number may carry appendages. In all cases the appendages primarily develop rami or branches which form the limbs, the primitive axis or corm being reduced and of insignificant size. In the most primitive stock all the post-oral appendages had gnathobasic outgrowths.
(f) The segmentation of the body is anomomeristic in the more archaic members of each class, nomomeristic in the higher members.