NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS I35 



compartments. The gonadial sacs of each lateral series united with 

 each other, forming thus a pair of tubular gonads, which opened to 

 the exterior through one pair of undivided coelomic sacs and their 

 outlet ducts. The ventral nephric sacs now became exclusively excre- 

 tory reservoirs, and, with the coelomoducts, formed a series of 

 nephridial organs along each side of the body. As a result of the 

 conversion of the original coelomic sacs into gonadial sacs and 

 nephridial sacs, the haemocoele was restored as the definitive body 

 cavity. 



At this stage of their evolution, the lobopod annelids assumed the 

 status of Protonychophora. Some of the protonychophorons retained 

 the flexible integument of the worms ; others developed a sclerotiza- 

 tion in the cuticula, and thus acquired an external skeleton of cuticular 

 plates. The soft-skinned forms, preserving some of the general 

 aspects of their annelidan ancestors, evolved into the modern Ony- 

 chophora ; the armored forms gave rise to the Protarthropoda. Since 

 the members of both groups were well adapted by their leglike appen- 

 dages to a walking mode of progression, many of their descendants 

 found an advantageous habitat on land. 



8. — The Onychophora retained the cylindrical wormlike form, but 

 they lost the segmented structure in the integument and musculature. 

 The lobiform appendages became more efficient locomotor organs 

 through the development of an incipient segmentation, and the acqui- 

 sition of terminal claws, but the first postoral appendages were con- 

 verted into a pair of jaws. The single pair of prostomial tentacles 

 took an apical position by migrating forward on the dorsal surface 

 of the head, but their nerve tracts were united by a commissure in the 

 posterior part of the brain. The eyes retained the annelid type of 

 structure. The somatic nerve cords, which presumably must have 

 been ganglionated in the segmented generalized annelids, became 

 simplified by a redistribution of the neurocytes, and took widely 

 separated positions along the sides of the body. The ganglia of the 

 jaw somite, however, united with the cerebral ganglion of the pro- 

 stomium and became posterior lobes of the brain. The coelomic sacs 

 of the penultimate somite, regardless of the total number of somites 

 in the body, were retained intact to serve as genital outlets ; the 

 persisting remnants of most of the other coelomic sacs became small 

 end-vesicles of the coelomoducts, which formed nephridial excretory 

 organs. 



p. — The Protarthropoda, because of the hardening of the integu- 

 mental cuticula, lost the flexibility and contractility of their annelidan 

 ancestors and onychophoran relatives, and, to compensate, developed 



