SECT, ix EXCRETORY AND OTHER GLANDS 133 



nephridia, whose arrangement in the Annelids them- 

 selves is always very variable. 



It at first appeared possible that the absence of 

 nephridia in Apus could be explained by assuming that 

 in the original Crustacean-Annelid they were developed 

 more in the posterior segments (which is in fact often 

 the case), and that these segments do not attain deve- 

 lopment in Apus, the enormous shell gland sufficing 

 for the removal of waste products from the blood. 

 The weakness of this argument is at once obvious. 

 It is only when all the segments are fairly well 

 developed that the permanent nephridia are limited 

 to the posterior segments. Nephridia or their rudi- 

 ments are, as a rule, to be found at one time or 

 another in the course of development in all the seg- 

 ments. As the posterior segments attain develop- 

 ment, the nephridia in the anterior segments often 

 disappear. Nephridia ought therefore certainly to be 

 found in the developed segments of the trunk of 

 Apus, and rudiments of nephridia in the larval 

 segments of which the posterior part of the 

 trunk of Apus is composed. Fortunately, we 

 are not driven to take refuge in such a doubtful 

 explanation. 



Knowing, on the one hand, that there are no true 

 nephridia in Apus, and on the other that in the carni- 

 vorous Annelids the nephridia are often the ducts for 

 the transmission of the sexual products, we naturally 

 tried to overcome the difficulty by the aid of the 

 genital glands. The study of these glands soon 

 yielded the desired clue. We have then here some- 



