NO. II LEGS OF PRIMITIVE ARTHROPODS EWING I7 



legs of certain genera. Its position as well as its rocking hinge and 

 attached levator muscle fibers indicate that it is the femur. The next 

 proximal segment is the largest of all and suggests a true femur. 

 However, its position in the leg series and its musculature indicate 

 that it is only a much enlarged trochanter. There remains the basal 

 segment, always short and stout and nearly always bearing an un- 

 segmented appendage, the stylus, at the base on its ventral side. This 

 segment should be regarded as the coxa, on account of its position 

 in the leg series, its musculature and its bearing of the so-called 

 stylus. In addition to these complete, or ring segments, there is a 

 chitinous plate of varying shape to which the coxa is attached and 

 on which the coxa moves by means of a rocking hinge. This is the 

 subcoxa. I have failed to find muscles attached to it although power- 

 ful depressor muscles arising from the postero-ventral angle of the 

 coxa pass inward to attach somewhere in the body. 



The segmentation here described at first seems to be out of harmony 

 with that of any of the other groups of primitive arthropods, but 

 when articulations and muscle attachments were studied, the very 

 unusual size and position of the femur became evident, thus making 

 the homology of the segments clear. Of all the leg segments the 

 femur is the least liable to be greatly reduced in size or to drop out 

 entirely, yet in the legs of symphylids apparently both of these things 

 have happened to it. 



In all the symphylids the first pair of legs has undergone a reduc- 

 tion in size. In Scolopcndrclla the front legs are about three-fourths 

 as long as the others and have the typical number of segments, /. e., 

 five. The shortening has been brought about by a big reduction in 

 the length of the second segment which is but slightly longer than 

 the third. In Scutigerclla and Hanscniella the front legs are further 

 reduced, but in these genera are only four-segmented (pi. 5, fig. 11). 

 Here the short third segment has either dropped out or has been 

 incorporated with the fourth which has lieen much reduced in length. 

 In Symphylclla only papillalike remnants are left of the front legs 

 which do not show segmentation. 



The symphylid leg, characterized by the presence of an appendage 

 on the coxa, a platelike subcoxa and a greatly reduced femur situated 

 at the bend of the leg, is probably nearer to the leg of Machilis than 

 to that of any other group. It differs from the leg of Machilis par- 

 ticularly in the small size of the femur and the large size of the 

 trochanter. 



