540 COMPARATIVE ANATOMY chap. 



relationship of the various orders united in this class. There is, further, no doubt 

 that those Arachnoidea whose bodies are most richly segmented have best preserved 

 the original character. These are the Seorpionidm, on account of the rich segmenta- 

 tion of their abdomen and abdominal nervous system, and the Solpugidce, on account 

 of the segmentation of that part of the body which answers to the cephalo-thorax of 

 other Arachnoidea ; this cephalo-thorax consists of an anterior section and three sub- 

 sequent segments, which may be compared with the head and three anterior trunk 

 segments of the AnUnnata. For such a comparison, however, further data as to the 

 structure and development of the Sol2>ugidce are needed. 



The Acarina, with their highly concentrated organisation, are evidently the 

 furthest removed from the racial form of the Arachnoidea. The other divisions in 

 various points take intermediate positions, and may be used as examples of pro- 

 gressive concentration. 



It is by no means proved that the Linguatulidce belong to the Arachnoidea. We 

 are not even in a position strictly to prove that they are Arthropoda. It is, in any 

 case, quite possible that the Linguatulidce are mite-like animals greatly modified by 

 parasitism, but they might just as well be degenerate descendants of other Arthropoda. 

 Definite data are wanting for deciding whether the want of a respiratory and blood- 

 vascular system is original, or has only arisen secondarily through parasitism. The 

 presence of 2 pairs of hooks does not prove that they are Arachnoidea or indeed Arthro- 

 poda at all. The history of their development also affords us no assistance. The 

 position of the female genital aperture at the posterior end of the body is unusual in 

 the Arachnoidea. The diverticula of the mid-gut so common among the Arachnoidea 

 are wanting here. The reduction of the central nervous system to an oesophageal 

 ring with several oesophageal ganglia is probably connected with the reduction of 

 sensory organs, extremities (1), etc., brought about by parasitism, but this does not 

 help to decide the question whether they are descended from Arachnoidea and not 

 from other animals provided with such a nervous system. There finally remains only 

 the constitution of the ovarial tubes beset with ovarian follicles, which specially recall 

 the Arachnoid arrangement, and opinions may differ as to the value of this point of 

 agi-eeraent. 



The question as to the systematic position of the Arachnoid class in the Arthro- 

 podan system is still a matter of discussion. There are two views on this subject. 

 According to one of these views the Arachnoidea are nearly related to the Xiphosura 

 and fossil Gigantostraca ; these three would then together form a third subphylum of the 

 Arthropoda, distinct both from the Crustacea and the Antennata. According to the 

 other view the Arachnoidea are racially connected with the Antennata, and form with 

 these and the Protracheata the subphylum of the Tracheata. At present we prefer the 

 latter view, and consider the Arachnoidea as Tracheata which have lost their antennfe,^ 

 while the first postoral pair of limbs, homologous with the mandibles of the Anten- 

 nata, has been pushed forward, so that in all adult Arachnoidea they are inserted in 

 front of the mouth. The cephalo-thorax of the Arachnoidea would then correspond 

 with the fused head and thorax (3 anterior tnmk segments) of the Antennata, and 

 we thus perhaps find in the segmentation of the cephalo-thorax in the Solpugidce a 

 primitive arrangement. If this supposition should prove correct, then the compari- 

 son of the other organs presents no very great difficulties. In judging of the system- 

 atic position of the Arachnoidea, their relationship to the Antennata is strongly 

 supported by the facts that the Arachnoidea possess both Malpighian vessels and 

 trachea3, which are wanting in the Crustaceans as in the Xiphosura. 



In the Antennata, the mandibles and the 2 pairs of limbs of the head which 



^ See footnote on page 516. 



