NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS 69 



motors, but are probably also levators and depressors of the leg as 

 a whole. Within the leg the fibers of the four somatic muscles spread 

 out into a thick peripheral layer of intrinsic leg fibers (fig. 31 C, D, 

 G, 75) attached on the successive rings of the thick basal part of the 

 appendage. Running through the narrow axial cavity of the leg is 

 an antero-posterior muscular septum (16), the fibers of which diverge 

 among those of the peripheral layer to the anterior and posterior walls 

 of the leg (C, D). The rest of the leg muscles, except a slender 

 transverse basal muscle (G, i/f), are motors of the distal rings and 

 of the claws. The former include a bundle of fibers (ly) arising 

 mesally in the leg base (G), with its fibers distributed to the ventral 

 walls of the distal rings (E, G), and a series of strong circular 

 muscles {18) in the pedal lobe. The claws are provided with a large 

 two-branched muscle (E, G, ig), the larger branch arising in the 

 base of the leg (G, ipa), the other in the distal part (E, G, ipb) ; 

 the short common terminal part is inserted dorsally between the bases 

 of the claws. The claw muscle is, therefore, a levator, or extensor, 

 of the claws and has no antagonist. 



It is quite reasonable to suppose that the onychophoran leg is a 

 prototype of the arthropod limb, but if we look for structural resem- 

 blances in these two sets of locomotor organs we find few such, if 

 any at all. The differentiation of the onychophoran leg into a thick 

 basal part and a slenderer distal part, and the individualization of the 

 distal rings, on which muscle branches are separately inserted, might 

 be seen as an incipient segmentation. There is, however, no actual 

 parallelism between the structure of the onychophoran leg and that 

 of any arthropod leg, so that all we can say of the former is that it 

 suggests a mode by which segmentation might arise in an ambulatory 

 appendage. We may conclude, therefore, that the appendages of the 

 Onychophora and the appendages of Arthropoda have had a common 

 origin as lobiform outgrowths of the body wall containing extensions 

 of the somatic muscles. The common need of a mechanism for an- 

 terior and posterior movement of each appendage on its base then 

 brought about a differentiation of the extrinsic parts of the limb 

 muscles into promotors and remotors, while the parts of the muscles 

 within the leg were elaborated to give greater efficiency to movements 

 of the leg itself. The further course of evolution producing segmen- 

 tation and correlated musculation in the limb evidently has proceeded 

 independently in the Onychophora and the Arthropoda from a very 

 primitive common beginning, and has gone much farther in the 

 Arthropoda than in the Onychophora. 



