NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS I4I 



Crustacea, it is possible that these structures were transmitted from 

 the Crustacea to the symphyhds and hexapods through the Proto- 

 myriapoda, though they have been lost in modern Diplopoda and 

 Chilopoda. The legs of the protomyriapods were all alike and retained 

 the generalized 7-segmented structure, but the extensor muscle of 

 the pretarsus was lost, leaving only the flexor muscle, which, for 

 more effective action, shifted its origin from the tarsus into more 

 proximal segments of the leg. This last feature is a distinctive char- 

 acter of all the descendants of the Protomyriapoda. N-ephridial excre- 

 tory organs were supplemented or replaced functionally by Malpighian 

 tubules of the proctodaeum. The position of the genital openings 

 was probably in general posterior, but variable. Postembryonic de- 

 velopment was anamorphic, the young being hatched with a small 

 number of segments, and the full number acquired by teloblastic 

 generation in the subterminal zone of growth. 



The Protomyriapoda undoubtedly were terrestrial, and the larger 

 forms may have developed tracheal invaginations on various parts of 

 the body for respiration, but there was no definitely established 

 tracheal system transmitted alike to all the descendent groups of 

 terrestrial mandibulates. The probable characters of the Proto- 

 myriapoda are summarized as follows by Imms (1936) : 



(i) The head bore a single pair of antennae and two pairs of jaws, viz. 

 mandibles and maxillae : the second maxillae were probably a subsequent acqui- 

 sition. (2) The trunk was composed of a variable and indefinite number of 

 sub-equal segments, each bearing a pair of legs. It is probable that anamorphosis 

 was universal and was continued throughout the life of the animal. (3) The 

 gonads opened to the exterior by paired apertures, and the segmental disposition 

 of the orifices probably varied in different families and depended upon that of 



the coelomoducts involved (4) The alimentary canal was probably a 



simple straight tube, while the excretory organs were little more than procto- 

 daeal outgrowths or pockets ; an accessory excretory function was probably 

 performed by the fat-body. (5) Respiration was probably cutaneous in many 

 forms and partially tracheate in others. The tracheae were presumably in the 

 form of groups of unbranched tubuli devoid of taenidia and bearing a general 

 resemblance to those of Diplopoda. Each group of tracheae opened laterally 

 by means of simple cryptlike, segmentally arranged spiracles : in some forms a 

 pair of spiracles was probably located also on the head. 



From the Protomyriapoda there emerged a specialized lateral 

 branch, the Protosymphyla, from which have been evolved in one 

 direction the progoneate modern Symphyla, Pauropoda, and Dip- 

 lopoda, in another the opisthogoneate Hexapoda, while the general- 

 ized myriapodan stock has more directly continued into the modern 

 Chilopoda. 



