NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS I3I 



THE GENITAL DUCTS 



The student of arthropod phylogeny is confronted at every turn 

 with the vexing problem that arises from the different position of 

 the genital outlets in the various arthropod groups, and in recent 

 years much discussion has been given to the question as to hovi^ the 

 heterogoneate condition came about (see Tillyard, 1930, 1932, 1935, 

 Snodgrass, 1933, 1936, Reynolds, 1935, Imms, 1936). Two phases 

 of the problem have been somewhat confused, namely, that pertaining 

 to the position of the openings of primary lateral ducts, and that 

 pertaining to the position of secondary median ducts. The opening 

 of a median duct is subject to migration, usually in a posterior 

 direction ; the openings of lateral ducts are closely associated with 

 particular segments, since the lateral ducts themselves represent 

 specific pairs of segmental coelomic sacs. 



The possible migration of lateral genital ducts is narrowly restricted 

 because of the limitations imposed by the transverse segmental nerve 

 trunks ; a secondary median duct formed by invagination of the ventral 

 wall of the body, however, lies beneath the ventral nerve cords, and 

 may, therefore, become lengthened from one segment to another by 

 an extension of its connection with the body wall. There is no evi- 

 dence to support Tillyard's (1930) contention that segmental gonads 

 were once connected by a common duct, which has retained a single 

 definitive opening on different segments in different arthropods, be- 

 cause when the germaria were segmentally arranged they were con- 

 tained in the dorsal parts of segmental coelomic sacs with individual 

 openings to the exterior, and the serial union of the dorsal parts of 

 the coelomic sacs has produced the definitive tubular gonads opening 

 through a single pair of coelomic sacs, while the ventral parts of the 

 other sacs discharging through the coelomoducts became nephridial 

 sacs. Likewise, Tillyard's (1935) second proposal that a heterogo- 

 neate condition has arisen by a variation in the number of somites 

 formed before or behind the primary genital somite cannot be accepted 

 for the reason that somite formation in the genital region is primitively 

 teloblastic. 



Inasmuch as the primary lateral genital ducts represent specific 

 coelomic sacs that have been retained to serve as genital outlets, a 

 segmental difference in the position of the genital openings is to be 

 explained only as the result of mutations that have been effective in 

 the organizer of the zone of teloblastic growth, which determines 

 what particular pair of coelomic sacs shall be utilized as genital exits. 

 A branching of the embryonic lateral ducts has been observed by 



