Morphology of the Arachnida. 3 



five somites are converted into tlie so-called tail ; tlic poste- 

 rior four pairs of appcndac?e.s of tlic abdominal re;^ion liave 

 disa])peared in connexion witli tlie development of the lung- 

 books, the second pair become tlie tactile sexual organs or 

 pectines, and tlic first in all probability constitute the genital 

 operculum *. Moreover the generative aperture has moved 

 forwards between the coxa: of the last pair of cephalothoracic 

 limbs, and the enlargement and ingrowth of the coxje of this 

 region have more or less obliterated the sternum. 



The Arachnida which structurally come nearest the 

 Scorpions are the Pedipalpi. There are three existing very 

 distinct types of this order, ThehjphonuSj Sckizonotus'\, and 

 Phryniis ; the first-named being the most Scorpion-like of 

 tlie three may advantageously be considered first. Great, 

 however, as is the superficial likeness between this genus and a 

 ►Scorpion, the diflerences are in reality very considerable. In 

 the first place thewholeabdomen isimmenselyreduced in length 

 by the shortening of the somites along the longitudinal axis, 

 the three posterior alone being abruptly narrowed to consti- 

 tute a small tail-like support for the filiform multiarticulated 

 telson ; in the second place a deep constriction separates the 

 cc])halotliorax from the abdomen. But more important than 

 all this is the disappearance of the two posterior lung-sacs and 

 the obliteration of the sternite and appendages of the second 

 abdominal somite by the enlargement and backward extension 

 of the sternite of the first, behind which the generative organ 

 opens. Moreover this first sternite, in addition to obliterating 

 the second, encroaches largely upon the third and fourth, 

 reducing them to narrow chitinous bands, the result being 

 that the pulmonary sacs that are situated in the third and 

 fourth somites open in front of their sterna, or, as it is usually 

 expressed, behind the first and second sterna +. 



* I am not aware tliat the evidence of the appendicular nature of the 

 genital operculum is absolutely conclusive. 



t A name proposed by Thorell to replace Kyctalops of Cambridge, 

 which was preoccupied. 



X Tliis at least seems to me to be the probable nature and extent of 

 the changes that have affected this region and these parts of the body. 

 I do not see otherwise how to account for the anomalous position of the 

 aperture of the pulmonary sacs behind the first and second sternites, when 

 tliese sacs belong to the third and fourth somites. 



If this view and the one expressed below as to the derivation of the 

 Aranea) from the Pedipalpi is correct, it seems that the two abdominal 

 sternites of tlie spider Liphistius and the opercula of the lung-sacs of the 

 Mygalomorphaj ai-e the homologues of the tirat and second sterna of the 

 Pedipalpi. In this case the anterior lung-sacs belong to the somite that is 

 represented by the second sternite in Lijihistius and by the second pair of 



