NO. IT LEGS OF PRIAflTIVE ARTHROPODS EWING 9 



spiders and the pseudoscorpions. In the spiders there are two short 

 basal segments, followed by the largest segment of the leg, then a 

 short and a long segment usually fused together, and finally two long 

 segments, the distal one bearing the claws. Comstock (1912) regards 

 these as follows : Coxa, trochanter, femur, patella, tibia, metatarsus 

 and tarsus respectively. 



In the pseudoscorpions, a leg typically consists of six segments in 

 addition to the pretarsus. However, in the genus Obisium all the 

 legs show seven segments. Borner (1921) has described the seg- 

 mentation and musculature of the leg in this group. In a typically 

 six-segmented leg there is a fixed platelike basal segment that meets 

 its fellow from the opposite side of the cephalothorax on the median 

 line. This basal segment is attached to the second, short, ringlike 

 segment with a double hinge set almost vertically. The third segment 

 is short and moves on a double horizontal hinge upon the second. 

 This third segment may be free or it may be immovably fused with 

 the fourth. When fused with the fourth (pi. 3, fig. 6, A), as it has 

 been in the hind legs of Chelifer, the fourth is much enlarged. Distal 

 to the fourth are two long slender segments and then the terminal 

 claws and pulvillus. Where the legs are seven-segmented there is a 

 suture in the last true segment beyond the muscle attachment (pi. 3, 

 fig.6,B). 



In the six-segmented leg, the last true segment (pi. 3, fig. 6, A) 

 giving rise to some of the claw-moving muscles is the last segment 

 of the leg. It should be regarded as the tibio-tarsus, as its derivation 

 through a union of the tibia and tarsus is made clear through a series 

 of intergrades. 



The next segment proximal, which is the first beyond the knee and 

 is always present and well developed, should be regarded as the patella. 

 Proximal to the patella is the femur. It is sometimes cylindrical and 

 sometimes enlarged, usually the latter in the case of the hind legs. 



The segment just proximal to the femur varies greatly. In the 

 hind legs of Chelifer (pi. 3, fig. 6, A) it is completely fused with the 

 femur, and has no special muscle inserted on it. In the first and 

 second legs, however, of the same genus, this segment is free, and 

 the femur is attached to it by a double hinge which works in almost 

 the same plane as the hinge at the base of this short segment. The 

 fact that this segment comes next to the femur and is frequently 

 fused with the same indicates that it is one of the trochanters. 



There remains the platelike basal segment and the one next to it 

 which is hinged to the short femoral trochanter. Does this platelike 



