126 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL.97 



which the three gnathal somites were intimately combined with the 

 protocephalon, while the appendages of the protocephalic somite 

 (second antennae) were suppressed. The Protomandibulata now be- 

 came Protomyriapoda. A composite head has thus been produced 

 along three separate lines of arthropod evolution, but in each case 

 with characteristic differences. 



The Protomyriapoda had all the characters common to the several 

 groups of arthropods finally derived from them, and also older 

 characters earlier transmitted to the Crustacea, which later appear in 

 one or more descendent groups, and are lost in others. From the 

 protomyriapods there arose the final persistent arthropod branch, the 

 Protosymphyla, while the main stem continued into the relatively 

 generalized modern Chilopoda (fig. 54). The Protosymphyla de- 

 veloped a labium by the union of the bases of the second maxillary 

 appendages, and so characteristic is this feature of all their descen- 

 dents, including the modern Symphyla, Diplopoda, Pauropoda, and 

 Hexapoda, that this group as a whole might be distinguished as the 

 Arthropoda labiata. The chilopods have developed few special fea- 

 tures other than the conversion of the first legs into a pair of poison 

 claws, but they have lost certain features of the protomyriapods. 



COELOMIC ORGANS OF ADULT ARTHROPODS 



In no modern adult arthropod is there retained a complete series 

 of coelomic sacs, but remnants of the coelom are preserved as the 

 lumina of the gonads and genital ducts, of various glands having an 

 excretory or accessory genital function, and perhaps of other glandu- 

 lar structures. The embryonic development of the coelomic sacs of 

 the Onychophora very probably recapitulates fairly closely the phylo- 

 genetic history of the coelomic sacs in both the Onychophora and 

 the Arthropoda. The primitive coelom of these animals undoubtedly 

 consisted of a full series of paired segmental cavities, each opening 

 to the exterior through a ventral diverticulum of the coelomic wall 

 connected with the ectoderm mesad of the base of the corresponding 

 appendage. The cavities must have served for the accumulation of 

 excretory products, and for the retention of the developing germ 

 cells, and the outlets gave vent to both the excreta and the 

 gametes (fig. 34 A). The more primitive annelids do not have per- 

 manent coelomic openings, and it seems doubtful that the simple 

 coelomoducts of the Onychophora had a common origin with the 

 metanephridia of the higher Annelida, since the metanephridia are 

 outgrowths of the posterior walls of the coelomic sacs and each opens 

 through the segment following. 



