ANATOMY OF ARACHNIDS 23 



tainly single,* but it lies in the anterior part of this segment and 

 extends anteriorly through the constriction into the last segment 

 of the abdomen in the egg just before hatching. I have not 

 had an opportunity of examining earUer stages. The suggestion 

 may be put forward that the last abdominal and first post- 

 abdominal segments represent in reality a single segment which 

 has become secondarily divided into two in order to give to the 

 base of the tail greater strength than could be obtained by 

 utilizing the di\'ision between two primary segments. This 

 suggestion seems to receive some support from Brauer's figures 

 of the development of the scorpion, a part of one of which is 

 reproduced in diagram 8. Brauer, in his text, treats segments 

 XIV and XV (first post-abdominal) as if they are two distinct 

 segments, but it appears from his figm^es as if segment XIV folds 

 over on itself to form the base of the tail, without at first any 

 segmentation at the point of the fold. 



That such a division is possible is indicated by the secondary 

 division between the fifth post-abdominal segment and the 

 poison gland, which corresponds to the telson of Limulus and 

 the Euiypterides. The telson is not considered as representing 

 an original segment, since it always lies behind the anus. There 

 is no ganglion for the poison gland in the scorpion. Similarly 

 the Uropygi and Koenenia have a tail jointed to the last abdom- 

 inal segment, but the tail is not recognized as a segment, nor 

 has it any corresponding ganglion. 



The scorpion-like Eurypterides of the Silurian have ten post- 

 genital segments — one less than the scorpion; the pre-genital 

 segment (VII) being recognizable on the dorsal surface, but on 

 the ventral surface it is fused with the genital plate. On the 

 other hand, the Silurian scorpion, Palaeophonus, appears to 

 have the same number of segments as the modern scorpions. 



The general conclusion is that in group I of the arachnids the 

 number of ganglia corresponds to the number of segments — 

 eighteen of each — obvious in the Pedipalps; not so clear, but 

 probable, in the scorpions; in the spiders possible but not proven. 



■* I have looked in vain through scores of specimens for some indication of 

 division of this ganglion into two neuromeres. 



