8 Mr. R. I. Pocock on the 



evidently segmented at least at its posterior end" *. Some 

 of the free-living Acari, e. g. the Trombidiidfe and Holothy- 

 rida^, with their pediforra, claw-ti})pcd palpi and chelate 

 mandibles, bear so strong arcscmblance to Sii/IoceUus, that with 

 an example of this genus in hand it is impossible not to 

 think that the Acari are descended from the Opiliones f. 



As for the Solifugae, their position is not so clear. They 

 seem to show affinities with the Pedipalpi through the 

 Tartaridcs (Schizonotidai) and also with the Pseudoscorpiones, 

 as Simon long ago suggested |. 



In the first place the carapace, instead of consisting of a 

 single shield, is usually described as being divided into three 

 distinct sclerites, the posterior two of which are regarded as 

 the tergites of the posterior two somites of the cephalothorax. 



The rest of the body, the abdomen, consists of only ten 

 somites, the tergites and sternites apparently corresponding 

 exactly in the first nine, while the last is a single plate 

 perforated mesially by a vertical slit — the anus. The genera- 

 tive aperture is, as usual, situated upon the ventral surface 

 of the first, while the second and third bear the apertures 

 of the tracheal breathing-organs. 



It is thus clear that these Arachnida fall into the same 

 section as the Pedipalpi, Aranere, Pseudoscorpiones, Opiliones, 

 and Acari, inasmuch as they are all devoid of the pectines 

 which are so characteristic of the Scorpions. So, too, does it 

 seem likely that the first sternite corresponds to that of the Pedi- 

 palpi, Araneje, &c., in which case it will represent, according 

 to our theory, the first enlarged abdominal sternite of the 

 Scorpion. Then the two following sternites bearing the 

 apertures of the breathing-organs will belong to the third and 

 fourth somites respectively, and the breathing-organs will 

 correspond in number and position with those of the Pedi- 

 palpi and Pseudoscorpiones. In this case one of the somites 

 has disappeared behind the fourth. In all probability the 

 missing one is the twelfth, and we can imagine that it has 

 vanished from view inside the eleventh, almost exactly as in the 

 Pseudoscorpiones. Then the eleventh somite will resemble 

 that of the last-named group in consisting of a single plate, 



♦ Bib. Sv. Vet.-Akad. Ilandl. xvii. no. 0, p. 8 (1892). 



t In an interesting paper, recently published in vol. xxiv. of the 

 Joum. Linn. Soc, Zooloi'V, my friend Mr. II. M. Bernard has advanced 

 reasons for showing that the Acari are derived from the Spiders. Without 

 now venturing to discuss in detail the views put funvard in this paper, 

 I will merely say that in my opinion the conclu^iuu arrived at would have 

 been nearer the truth if the word " Opilionid " had been substituted all 

 throusrh for " Araneid." 



X 'IjCS Arachnides de France/ vii. pp. 9, 10 (1879). 



