XVII 



EXTERNAL FEATURES 



443 



is long and narrow in the Mecostethi, and Cyphophthalmi, but 



in the Plagiostethi, which include most of the forms found in 



temperate regions, it is very short 



and transverse, and is hidden by 



the abdominal prolongation before 



mentioned. 



The anterior wall of the mouth 

 is formed by a beak-like plate, the 

 " epistome," the basal portion of 

 which is covered externally by a 

 second plate, for which Simon l pro- 

 poses the name " pre-epistome." In 

 some Phalangids there are three little 

 chitinous plates, one median and two 

 lateral, on the clypeus, between the 

 anterior border of the carapace and 

 the insertion of the chelicerae. They 

 are best seen in Nemastoma. 



The abdomen always presents 

 evidences of segmentation, though 

 there is a difference of opinion as to the number of segments 

 of which it is composed. This is due to the already mentioned 

 partial or complete fusion of the anterior segments with the 

 cephalothorax. From the admirable researches of Hansen and 

 Sorensen 2 it seems likely that the normal number of abdominal 

 segments is ten. Ventrally, the abdomen is produced forward 

 into a " sternal process " which is capped by a genital plate, 

 hardly distinguishable in the 1'halangidae, but readily visible 

 in the other families, which surrounds and masks the unpaired 

 genital orifice. Two stigmata or breathing pores are situated on 

 the sides of the first ventral plate, which these authors consider 

 to be composed of two fused stern ites. 



As in other Arachnids there are six pairs of appendages 

 articulated to the cephalothorax. They are the chelicerae, the 

 pedipalpi, and the four pairs of ambulatory legs. 



The chelicerae are three-jointed and chelate, the second joint 

 having its inner portion produced into an apophysis to which 

 the final joint is apposed. In certain forms (Gonyleptidae, 



Fi<;. 231. Mouth-parts of Fhalan- 

 gium. A, B, C, Gnathobases of 

 pedipalp and first and second 

 legs ; ch, chelicera ; ep, epistome ; 

 lab, labium ; m, mouth ; ped, 

 pedipalp ; pre.ep, pre-epistome ; 

 st, sternum, shown by the removal 

 of the anterior part of the genital 

 process, which extends to the 

 dotted line ; 1, 2, 3, 4, legs. 



1 Arachnides de France, vii., 1879, p. 122. 

 2 On two Orders of Arachnida, Cambridge University Press, 1904. 



