STRUCTUEE AND CLASSIFICATION OF THE ARTHEOPODA. 565 



including- the pi'ostliomere^ in wliicli alone it atrophies in latei- 

 development. 



(j) The venti'al nerve-cords are widely separated^ — in fact, 

 lateral in position. 



(k) There are no masses of nerve-cells forming a ganglion 

 (neuromere) in each somite. (In this respect the Protarthro- 

 poda are at a lower stage than most of the existing Chasto- 

 poda.) 



(/) The genital ducts are formed by the enlargement of the 

 coelomoducts of the penultimate somite. 



Class (Unica) . — Ontchophora. 



With the characters of the grade : add the presence within 

 the body of fine nnbranched tracheal tubes, devoid of spiral 

 thickening, opening to the exterior by numerous irregularly 

 scattered tracheal pits. 



Genera — Eoperipatus, Peripatopsis, Opisthopatus, etc. 



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 joijited pieces. 



{})) 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, tritoguathous, or 

 tetartognathons. 



{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 primarll^'^ develop rami or branches 



