STRUCTURE AND CriASSTFIOATION OF THE AKTHROPODA. 567 



Somites, auomomeristiCj from 12 to 150 in the post-cephalic 

 series. 



The genital dncts open in the fourth, or between the fourth 

 and fifth post-oral somite. 



Terrestrial forms with small-jointed legs formed by adapta- 

 tion of a single ramus of the appendage. Trachege are 

 present. 



Note. — The Diplopoda include the Juliformia, the Sym- 

 phyla (Scolopendrella), and Pauropoda (Pauropus). They 

 were until recently classified with the Chilopoda (centipedes), 

 with which they have no close afiinity, but only a superficial 

 resemblance. (Compare the definition of the class Chilopoda.) 



The movement of the legs in Diplopoda is like that of those 

 of Peripatus, of the Phyllopod Crustacea, and of the para- 

 podia of Chgetopoda, symmetrical and identical on the two 

 sides of the body. The legs of Chilopoda move in alternating 

 groups on the two sides of the body ; this implies a veiy 

 much higher development of nerves and muscles in that group.^ 



Class 2 (of the Euarthropoda). — Arachnida. 



Head tritoguathous and diprosthomerous, — that is to say, 

 with two prosthomeres ; the first bearing typical eyes, the 

 second a pair of appendages reduced to a single ramus, which 

 is in more primitive forms antenniform, in higher forms 

 chelate or retrovert. The ancestral stock was pantognatho- 

 basic, i. e. had a guathobase or jaw process on evei'y para- 

 podium. As many as six pairs of appendages following the 

 mouth may have an enlarged guathobase actually functional 

 as a jaw or hemignath, but a ramus is well developed on each 

 of these appendages either as a simple walking leg, a palp, 

 or a chela. In the more primitive forms the appendage of 

 every post-oral somite has a guathobase and two rami; in 

 higher specialised forms the gnathobases maybe atrophied in 

 every appendage, even in the first post-oral. 



* See the Appeiulix at the end of the present article, and the accompanying 

 plate. 



