20 B. H. BUXTON 



Pedipalps 



In the Pedipalps, both Uropygi and Amblypygi, the relations 

 are clear enough. There is a small first abdominal segment 

 (VII), after which comes the second abdominal, or genital seg- 

 ment (VIII) ; posterior to the genital segment are ten more 

 segments, making twelve abdominal segments in all; the num- 

 ber of segments therefore corresponding to the number of gan- 

 glia — six for the cephalothorax and twelve for the abdomen. 



Spiders 



In the ordinary spiders all trace of segmentation of the abdo- 

 men is lost, but the pedicle is taken to represent a first abdomi- 

 nal segment (VII) after which comes the genital organ repre- 

 senting segment VIII. Liphistius has nine tergal plates on the 

 abdomen, each of which is presumed to represent a segment, so 

 there are still three segments wanting. McBride ('14), quoting 

 Kishinouye, says that in the earlier stages of development of 

 Theridion ten segments are marked out in the abdomen, of 

 which the tenth is not always distinct, nor does it always de- 

 velop a coelomic cavity. There is no proof, therefore, of corre- 

 spondence between the number of ganglia and segments in the 

 spiders, although, since the rudiment of the tenth abdominal 

 segment is not always laid down, it is possible that an eleventh 

 and a twelfth segment may have originally existed but have now 

 entirely disappeared. 



Scorpions 



There is not complete correspondence between the ganglia 

 and the segments in the scorpions, although the total number 

 of eighteen is the same. The cephalothorax is composed of 

 six segments as usual, and the abdomen of twelve, but the first 

 abdominal segment (VII) has disappeared, leaving the genital 

 as the first abdominal segment, posterior to which are the seg- 

 ments bearing the pectens, the four lungs and the last abdomi- 

 nal segment — making seven in all — while beyond this comes the 

 post-abdomen with five segments forming the tail. There are, 



