314 ME. H. M. BERNAED ON THE 



motion and prehension, is followed by a well-demarcated abdominal region specialized as a distensible 

 sac-like receptacle for digestion and reproduction. This specialization comprises all the abdominal 

 segments in Galeodidse, Pseudoscorpionidse, Araneidse, Phalangidae, Acaridse, while in the Scorpionidse, 

 Thelyphonidse, and Schizonotidje a few of the most posterior segments form together a kind of caudal 

 appendage secondarily specialized for different purposes. In all discussions as to the primitive segmen- 

 tation of the abdomen, we have to bear in mind that its swollen condition is a secondary specialization. 

 In Scorpio and Thehjphomis, although the posterior segments of the abdomen take no part in the 

 formation of the visceral sac, they are greatly specialized for other functions. Moreover, these tail- 

 formations difPer too greatly to be considered primitive structures. The lengthening of the tail- 

 segments of Scorpio was most probably a secondary and compensatory specialization to allow of the 

 use of the terminal sting, as the anterior abdominal segments were becoming sac-like and thus no longer 

 capable of bending backward. 



We may, I think, safely conclude from the varied conditions of the abdominal segmentation of existing 

 Arachnids that the abdominal segments of the ancestral form were quite unspecialized. The differences in 

 the number of the segments specialized for distension shows clearly that neither this nor the " tail " 

 is primitive. In Scorpio, seven form the distensible sac and five the tail. In Thelyphonus, nine form 

 the distensible sac and three a totally different tail ; while in Galeodes ten, in Cliernes jierhaps eleven, 

 form the distensible sac, no segments at all being specialized into a tail. It is impossible to deduce these 

 forms from any specialized ancestor. 



The reduction in the number of segments in some Arachnids is doubtless in some way due to the 

 excessive weight of the distended sac. 



We have thus tolerably clear evidence that the original and most important specialization of the 

 Arachnidan abdomen was into a digestive and reproductive sac, while the anterior region was specialized 

 for locomotion. 



Diaphragm or Waist. — Between the 6th and 7th segments, i. e. between the cephalo- 

 thorax and abdomen, there is, in the Galeodidae, no true specialized waist, although there 

 is a strong intersegmental constriction, the abdomen being able to bend up almost at right 

 angles to the cephalothorax at this point. Internally, this constriction has given rise 

 to a very striking " diaphragm," which has already been described and figured by Kittary 

 (41). It forms a very complete wall between the interior of the cephalothorax and that 

 of the abdomen, and is pierced by the dorsal vessel, the alimentary canal, the nerve- 

 cords, and the tracheae. Close examination shows that this diaphragm is due, as 

 stated, to a strong indrawing of the intersegmental membrane between the above- 

 mentioned segments, so that it is composed partly of a chitinous infolding and partly 

 of muscle-bands. 



It is clear that if the opposite two internal faces of such a deep segmental constriction 

 fuse together, they form a diaphragm : if they remain unfused, they form a waist. In 

 the Galeodidse we seem to have an unsj)ecialized ai'rangement, the intersegmental 

 infolding being fused only in its deeper parts, forming the diaphragm, while the outer 

 parts of the fold remain open, making some approach to a waist. 



Waists or diaphragms appear to be characteristic of the Arachnids. The Pedipalpi and the Aranese 

 have highly specialized waists. Scorpio has secondarily lost all traces of its waist and has only a 

 diaphragm. The Pseudoscorpions show very deep constrictions laterally and ventrally, but not dor- 

 sally ; these constrictions thus do not form a true waist. The Phalangidie and Acari are the only forms 

 which appear to have lost all traces of the former waist. This is perhaps to be correlated with the 

 diminution of the abdominal region characteristic of the former and of many of the latter. 



