NO. II LEGS OF PRIMITIVE ARTHROPODS EWING I3 



the flexor of the claws. This muscle is attached to the claw base. It 

 can be traced backward as a minute, poorly chitinized tendon which 

 passes very soon into the strongly chitinized end of the tendon of 

 the muscle proper (pi. 4, fig. 7) . As the muscle is followed proximally 

 through the tarsus it enlarges until about half of the area of a cross 

 section is occupied. The most of the fibers attach dorsally to the wall 

 of the tibia, but in the second and third pairs of legs some strong 

 strands extend into the femur attaching dorsally near the tip of this 

 segment. In neither AccrcntuJus harheri or Eosent onion vcnniforme 

 or any of the other American species examined were muscle attach- 

 ments found on the tarsus. The next (proximally) muscle is the 

 powerful flexor of the tarsus. It arises, as far as could be ascertained, 

 entirely from the femur. The flexor of the tibia follows. It has also a 

 powerful muscle arising from the chitinous ring of the 1)ase of the 

 trochanter and attaching to the ventral side of the base of the tibia. 

 Prell (1912) finds extensor muscles attached to the femur. In Accr- 

 ciitiilus harheri such a muscle could not be detected. However, a small 

 flexor muscle was found as represented by Berlese (1910) for Accr- 

 cntomon dodcroi except that it arises from the base of the trochanter 

 instead of the base of the coxa. Two powerful muscles are situated 

 in the coxa, one a levator of the trochanter and the other a depressor. 

 The coxa, which is hinged above to the lower sclerite of the subcoxa 

 and below to the sternum, is rotated by two powerful muscles arising 

 from the dorsal wall of the thorax, the anterior being the protractor 

 of the coxa and the posterior the retractor of the same. 



Snodgrass (1927) has emphasized the similarity between the mus- 

 culature of the proturan leg and that of a caterpillar. These resem- 

 blances have to do with the simplicity of the musculature, the almost 

 complete absence of extensor muscles and the passing of strands of 

 several muscles through more than one segment. In the leg of a 

 caterpillar the flexor muscle of the claw sends some fibers to attach 

 to the base of the coxa. Here, therefore, is an instance of a muscle 

 passing through the entire length of a leg. 



The presumption is frequently made, and doubtless with reason, 

 that in the leg of a proturan and that of an insect larva we have not 

 only a simple type of mu.sculature but also a primitive one. If this 

 is a primitive insectan type, and the present writer believes it is, 

 it is quite dififerent from the primitive type for the Arachnida. In the 

 Solpugida and other primitive arachnids the legs are composed of a 

 large number of segments and but few of the muscles pass through 

 more than one segment, and there are several extensors. The 

 Arachnida are believed to have had their descent from ancestors 



