512 



COMPARATIVE ANATOMY 



CHAP. 



In the body of the Arachnoidea we find almost always a number of 

 anterior segments, probably 7, fused to form a generally unsegmented 

 eephalo- thorax. Following upon this cephalo- thorax there is an 

 abdomen consisting of a varying number of separate or fused segments, 

 which may again fuse with the cephalo-thorax, as is the case in the 

 Acarina and Linguatulidce ($) and thus the body appears neither segmented 

 nor divided into regions. We thus, within the class of the Arachnoidea, 

 have not only fusing of segments and an obliteration of segmentation, 

 but also a progressive concentration of the whole body ; there can be 

 little doubt that the Arachnoidea are no exception to the rule that the 

 more richly and completely segmented the body is the better has it 

 retained the primitive arrangement. The segmentation is richest in 



the Scorjrionida' and Solpngidce, but 

 is very different in the two groups. 

 The segmentation in both these 

 forms claims special attention. 



The Solpugidic (Fig. 3 5 8) vividly 

 recall the Insecta in the metamerism 

 of the body. In opposition to all 

 other Arachnoidea, not only is the 

 anterior division of the body, 

 answering to the cephalo-thorax, 

 distinctly segmented, but it even 

 falls into two parts, an anterior 

 unsegmented head, which may be 

 compared with the head of the 

 Antennata, and a posterior thorax, 

 consisting of three segments, which 

 may be compared with the thorax 

 of the Inwta and with the three 

 anterior trunk segments of the 

 Mi/riapnda. An abdomen of ten 

 segments follows the thorax. 



There are considerable difficulties in 

 the way of comparing the segments of the 

 body of the Sulpiujida with the head and 

 a corresponding number of trunk segments 

 of the A-ntciinata ; these difficulties arise 

 chiefly in comparing the. extremities and 

 nervous system, and will lie discussed later 

 on. The ontogeny of the Solpugidm is 



unfortunatclv almost unknown. 



i_'. :;:/.'. Scorpio africanus (;dlur Cuvier, 

 J;JIH animal). 



Iii the Scorpionidw (Fig. 359) 

 the. eephalo-thorax is, in the adult 

 condition, unsegmented; in the 



embryo, however, a segmentation into seven somites, including the 

 frontal lobes, may be recognised. The abdomen, on the contrary, 



