Morphology of the Arachnida. 11 



their true systematic importance cannot be imclerstood. For 

 it is most interesting to note that Stylocellus, which is a true 

 Opilio, possesses two eyes on each side of the carapace, one 

 raised upon a tubercle, the other at the base of the tubercle. 

 This last eye has disappeared in the closely allied genus 

 Pettalus, which thus resembles the higher Opiliones in being 

 furnished with oidy two eyes, but whether or not they are 

 homologous to the two eyes of a Phalnngium, I am not able 

 to say. If they are not, the fact will constitute a radical 

 diflerence between the Sironidte {Siro^ Pettalus, Stylocellus), 

 constituting Thorell's suborder Anepignathi, and his Lania- 

 tores and Palpatores. Certainly the two eyes of Pettalus from 

 their j)osition strongly call to mind those of the Pseudo- 

 scorpiones and of the unplaced genus Gibbocellum of Stecker. 

 But until the exact nature of all these eyes has been deter- 

 mined by a study of their development and minute structure, 

 no very great taxonomic value can safely be placed upon 

 them. 



Having thus passed in review the most important external 

 organs of the Arachnida, it seems to me that the best characters 

 for the classification of the class are to be found in the 

 abdomen. 



The following grouping of the orders of Arachnida will 

 serve to show briefly my present views as to their affinities : — 



A. The embryo provided with .six pairs of abdominal 



appendages, the second of which persists in the 

 aduh as the pectiues. The adult with four pairs 

 of abdominal breathing-organs in the form of 

 lamellar trachese ; the abdomen very long, the 

 posterior five segments compressed to form a 

 tiexible tail; the post-anal sclerite fiu-nished Subcla-s 



with two poison-glands ; viviparous Ctenophora. 



Order 1. Scorpioxks. 



B. The embryo not provided with more than four 



pairs of abdominal appendage?, the second of 

 which are never retained as external organs in 

 the adult. Not more than two pairs of abdo- 

 minal breathing-organ?. Post-anal sclerite 

 usually ab-ent and never provided with poison- 

 glands ; abdomen much shorter, with at most 

 the three poiterior segments narrowed to form Subclass 



a tail, usually oviparous Lipoctena. 



A. Cephalothorax and abdomen separated by a 

 deep constriction ; the first abdominal stomite 

 or its remnants covering the apertures of thi' 

 generative organs and of the first pair of re- 

 spiratory stigmata. The breathing-organs, 

 except in some of the Arachnomorphuus spi- 

 ders, in the form of lamellar trachese ; with 



