ATOPETHOLID MILLIPEDS — HOFFMAN AND ORCUTT 109 



apodeme, doubtless aiding in drawing the gonopod down and mesiad, 

 and freeing it from the gonocoel. 



8. A large thick muscle originating on the anterior sternal apodeme 

 and inserting on the septum on the inner side of the vinculum. The 

 homologs of this muscle may be found in the sternal areas of platydes- 

 moid and chordeumoid millipeds, where the median septum is re- 

 flected externally by a median sternal ridge or knob. 



9. A short band of muscle from the anterior sternal apodeme to the 

 caudal side of the coxal apodeme. The muscle serves to swing the 

 coxite slightly mesiad in contraction, in opposition to the action of 8. 



It should be noted that the sum effect of the motor activities of the 

 gonopods is that of a clasping organ. A hypothetical reconstruction of 

 the procedure is submitted at this time to invoke the interest of other 

 investigators. So far nothing has been published on the mechanics of 

 copulation in this order, and direct observations are needed to estab- 

 lish the function of the gonopods. 



Initially, the entire gonopod apparatus is at least partially extruded 

 by the protractor muscles. The coxites are then spread slightly laterad 

 by contraction of 8. Muscles 6 and 7 can then pull the posterior 

 gonopod out of the gonocoel. This done, they relax, and the entire 

 posterior gonopod is exserted by the contraction of 4 until the append- 

 age is exposed and probably directed toward the head of the animal. 

 At this point muscles 1 and 2 pull the gonopod so that the distal end 

 swings down toward the bases of the walking legs, a position enhancing 

 contact with the female cyphopods. Retraction is made by muscles 

 3,6, and 7, with 6 and 4 then cooperating to return the appendage into 

 the gonocoel. 



Sympleurtte of seventh segment: In spu'oboloid diplopods, the 

 two pairs of appendages of the seventh segment are modified as 

 gonopods and withdrawn into the body cavity, and thus remove the 

 two sternal elements from the normal segmental ring. The pleurites of 

 the segment project mesiad to fuse at the mid ventral line and main- 

 tain segmental rigidity. In some families, such as the Rhinocricidae, 

 there is no visible midline suture, and the combined pleurites are 

 modified into an elevated thin flange that projects somewhat caudad 

 and protects the exposed tips of the posterior gonopods. Most of the 

 atopetholids, however, retain the median suture and the modified 

 pleurites — herein designated as sympleurite — form a simple transverse 

 bar. An exception occurs in the subfamily Arinolinae, where the 

 sympleurite is medially enlarged and variously lobed cephalad, as 

 shown in figure 11, d. Although no good comparative study could be 

 made in this connection, it appears that each species may differ some- 

 what in the shape of the sympleurite at least in the genus Arinolus. 



Legs: In most instances the leg joints maintain a uniform propor- 

 tion and appearance, the order of decreasing length usually 3, 6, 2, 4, 5, 



