SECT. IX EXCRETORY AND OTHER GLANDS 137 



(5) The position of the only aperture which re- 

 mains agrees well with the position of the opening 

 of the Annelidan nephridium, i.e., it lies laterally on 

 the ventral surface. The fact that, in Apus, it comes 

 between two consecutive limbs is due to the bending 

 round of the parapodial limbs towards the ventral 

 middle line as already described (see § on Append- 

 ages). 



(6) The genital aperture does not always remain 

 in the same segment in the Crustacea ; it differs even 

 in the males and females of the same species. 



(7) Lastly, we have to add the fact that the genital 

 ducts have been generally recognised as homologous 

 with Annelidan nephridia. 



We thus believe that though the nephridia are ab- 

 sent in Apus, we have found sufficient traces of their 

 having been once present in the typical manner, one 

 pair in each segment, functioning, as they do typically 

 in Annelids, as ducts for the transmission of the sexual 

 products. The great development of the shell gland 

 rendering the excretory functions unnecessary, there 

 remained only the secondary function of conduct- 

 ing the sexual products out of the body. As this 

 could be done equally well by simple tubes formed 

 out of the covering of coelom epithelium, these latter 

 alone have been retained, preserving, however, the seg- 

 mental arrangement and the position of the nephridia 

 which they had at one time clothed. 



These considerations seem sufficient not only to 

 remove the difficulty caused by the absence of ne- 

 phridia in Apus, but even to strengthen the evidence 



