Morphology oj the Arachnida. ^ 



five somites are converted into the so-called tail ; the jDOSte- 

 rior four pairs of appendages of the abdominal region liave 

 disappeared in connexion with the development of the lung- 

 books, the second pair become the tactile sexual organs or 

 pectines, and the first in all probability constitute the genital 

 operculum *. Moreover the generative aperture has moved 

 forwards between the coxaj of the last pair of cephalothoracic 

 limbs, and the enlargement and ingrowth of the coxee 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, Thelyphonus^ ScMzonotus^^ and 

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

 the three may advantageously be considered first. Great, 

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

 Scorpion, the differences are in reality very considerable. In 

 the first place the whole abdomen is immensely reduced 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 

 tclson ; in the second place a deep constriction separates the 

 cephalothorax 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 cliitinous bands^ the result being 

 that the pulmonary sacs that are situated in the third and 

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

 expressed, behind the first and second sterna J. 



* 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 Nydalops of Cambridge, 

 which was preoccupied. 



X This 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 steruites, when 

 these sacs belong to the third and fourth somites. 



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

 Araneie from the Pedipalpi is correct, it seems that the two abdominal 

 sternites of the spider LipJiistius and the opercula of the lung-sacs of the 

 Mygalomorphse are the homologues of the first 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 Liphistiiis and by the second pair of 



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