CHAPTER XV 



THE SUBPHYLUM ARACHNIDA 



Arthropods with fully chitinized exoskeleton ; the anterior part of the 

 body (prosoma), never divided into head and thorax, consisting of six 

 adult segments, the first (preoral) with prehensile appendages 

 (chelicerae) usually three-jointed, the second (postoral) with append- 

 ages either sensory or prehensile (pedipalps) and the remaining four 

 ambulatory ; the posterior part (opisthosoma) consisting of thirteen 

 segments and a telson in the most primitive forms but tending to 

 become shortened, the first (pregenital) segment differing from the 

 rest, the second bearing the genital opening; respiratory mechanisms 

 of various types usually developed in the anterior part of the opistho- 

 soma ; coxal glands of coelomic origin in the 2nd to 5th prosomatic 

 segments ; larval forms absent except in Limulus. 



As has been pointed out in the introduction to the Arthropoda, the 

 Arachnida are distinctly marked off from the rest of the phylum by 

 the character of their appendages and especially by their chelicerae 

 which furnish so strong a contrast to the sensory antennae, elsewhere 

 found in the phylum. Moreover, nowhere else (except perhaps in 

 trilobites) are true jaws absent, the prolongation of the basal joint of 

 the anterior limbs toward the mouth (gnathobases) serving the arach- 

 nids for mastication. In the divisions of the group is found the greatest 

 diversity in form, for though by no means active creatures, arachnids 

 have become adapted to many kinds of environment. 



Besides the segments enumerated in the preamble, there is in the 

 embryo of most arachnids a precheliceral segment (Fig. 352 B, C). 

 The variation in the segments of the prosoma is confined to minor 

 details, the chelicera preserving much the same characters throughout 

 the group, only losing a joint in the Araneae, and being either chelate 

 or subchelate ; the pedipalp, however, varies according to its function, 

 being chelate in the scorpion and the Pedipalpi, which seize their prey 

 by means of it, modified for purposes of fertilization in the spiders, 

 and merely an ambulatory appendage in Limulus. In most forms 

 the tergites of the segments are fused together, but in the Pedipalpi 

 and the Solifugae the last two prosomatic segments are entirely free. 



It is in the opisthosoma and its segments that the greatest amount 

 of variation can be seen. The pregenital segment (Fig. 352 B, C) is 

 always developed in the embryo, but tends to disappear in the adult. 

 Thus in the Palpigradi, Pedipalpi and Pseudoscorpionidea it forms 

 a distinct segment; in Limulus it is represented by a pair of rudi- 



