NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA — SNODGRASS 6/ 



each two successive legs, therefore, the relation of the two sets of 

 oblique fibers is reversed. On the venter all the fibers again take the 

 same relative position that they have on the back. The two sets of 

 oblique fibers arise on the integument close to the middorsal and 

 midventral lines, and are hence not continuous from one side to the 

 other. External to the oblique fibers may be seen the anterior and 

 posterior ventral muscles of the legs (fig. 30, right, 11, 12). 



Finally, outside all the other muscles of the body wall, are the 

 circular muscles (fig. 30, /?). They consist of extremely fine fibers 

 closely adherent to the inner surface of the integument, and are 

 apparently continuous across the middorsal and midventral lines. 



A few other body muscles occur in the region of the mouth, and 

 the jaws have an elaborate musculature quite different from the 

 musculature of the legs (fig. 21 F). 



THE SEGMENTAL APPENDAGES 



The appendages of the Onychophora include the antennae, the 

 jaws, the oral papillae, and the legs. Their rudiments appear in the 

 embryo as conical outgrowths of the body wall (fig. 23). The an- 

 tennae arise from the anterior angles of the cephalic lobes (B, Ant) 

 and retain this position. The jaws, which are the appendages of the 

 first postoral somite, arise posterior to the mouth (A, /), but later 

 they migrate mesally and forward (B), and are finally buried in the 

 preoral mouth cavity (C), where they become reduced to a pair of 

 double flattened hooks (fig. 21 F) converging in a horizontal plane 

 beneath the mouth (D, /). The oral papillae are the appendages of 

 the second postoral somite (fig. 23 A, OP), but in the definitive state 

 they take a more anterior position at the sides of the mouth (fig. 21 D, 

 E, OP). The legs retain their primary lateroventral positions (fig. 

 23 B, C), and show but little variation in their final structure. 



The onychophoran appendages in their development give no evi- 

 dence of having been derived from polychaete parapodia; they have 

 no cirri or bristle sacs, and nothing suggests that they are composite 

 organs formed of notopodial and neuropodial elements. The terminal 

 claws of the onychophoran leg in no way resemble parapodial chaetae, 

 and the general structure and musculature of the leg has little in 

 common with a parapodium. except features that adapt each appen- 

 dage to forward and backward movement on its base. On the other 

 hand, the segmental appendages of the Onychophora and the Arthrop- 

 oda have the same manner of origin and growth in the embryo, the 

 organs in each case being hollow musculated lobes of the body wall, 



