NO. I INSECT THORAX — SNODGRASS 85 



its movements are unrestricted, it is provided with antagonistic sets 

 of muscles corresponding with its three primary axes of motion. 

 It has promotors and rcmotors (fig. 37 A, I, J), abductors and ad- 

 ductors {M , N), and anterior and posterior rotators (K, L). 



If the coxa represents the base of the primitive arthropod Hmb, 

 we have only to assume that its muscles were directly fitted to its 

 needs. We have seen, however, that there is reason for believing that 

 the original limb had its base in a subcoxal segment, which became 

 incorporated into the pleural wall of the body to form a support for 

 the rest of the limb, the latter acquiring a new functional base in 

 the coxa. This theory can not be supported on external features alone; 

 the transformations that it assumes could not but involve changes 

 in the leg musculature, and it must be shown at least that there is 

 nothing in the arrangement of the muscles in modern insects that is 

 incompatible with the theory — if positive evidence can be found of 

 shiftings in the muscle attachments in accord with the assumed 

 changes in the skeletal parts, the theory will be so much the more 

 acceptable, 



A primitive segmental limb, perhaps of a parapodial nature, must 

 have turned forward and backward on its base in the lateral wall of 

 its segment. It, therefore, possessed promotor and remotor muscles, 

 probably dorsal and ventral promotors (fig. 38, A, I, K) and dorsal 

 and ventral remotors (J, L). According to Borner (1921) the para- 

 podium of an annelid worm (Nereis) has a musculature of this sort 

 by which it is turned anteriorly and posteriorly. If, then, the arthropod 

 subcoxa turned on a vertical axis, it follows that the coxal movement 

 on the subcoxa was most probably in a vertical plane on a horizontal 

 axis. The coxa, therefore, had abductor and adductor muscles 

 (M, N) arising on the dorsal and the ventral wall of the subcoxa. 



Something analogous at least to this theoretical musculature of the 

 primitive limb base (fig. 38 A) may be seen in the Acarina. In 

 the IxodidcC the movable part of each leg is supported on a large 

 basis that spreads out as a wide plate in the ventral wall of the body 

 (fig. 17, Sex), but which also is narrowly continuous around the 

 dorsal side of the first free segment of the leg. These leg bases are 

 provided with anterior and posterior dorsal muscles, and the bases 

 of the first pair of legs in Dermacentor and Amblyomma at least 

 are slightly movable in the living tick, turning on an obliquely trans- 

 verse axis. The next piece of the leg is a free, cylindrical segment 

 (Cx), hinged to the basis by anterior and posterior articulations on 

 an axis at right angles to that of the basis with the body, the anterior 



