NOTES ON THE ANATOMY OP DINOPHILUS. 51 



external, is situated, in the adult condition, in front, tlie whole organ 

 having now acquired an antero-posterior direction. The funnel, 

 during the above changes, will naturally come to be situated near 

 the posterior end of the organ. 



There seems, therefore, fair reason to assume that the young 

 vesicula seminalis shown in fig. 5 is morphologically the fifth 

 nephridium ; it must be especially noted that the funnel of the vesi- 

 cula is in a position corresponding with that of the ciliated appendage 

 of an ordinary nephridium, and that the original external aperture 

 of the modified nephridium was probably (in the phylogenetic history 

 of the organ) at the opposite end of the tube, which ultimately be- 

 comes the blind anterior end of the vesicula. The relations of the 

 outer end of the young vesicula to the ciliated rings of the fifth seg- 

 ment further support this conclusion. The connection of the vesicula 

 seminalis with the penis would, in this case, have to be regarded as 

 having been acquired secondarily. Should the above account of the 

 vesiculse seminales of D. tseniatus be confirmed, the structure and 

 mode of origin of these organs might be held to have an important 

 bearing on the question of the phylogeny of the diiferentiated 

 Chaetopod nephridium. The structure of the first four nephridia in 

 the male D. tseniatus, or of all five nephridia in the female, is obvi- 

 ously comparable with that of the head-kidney of a Chaetopod larva. 

 In this connection the figures given by Ed. Meyer (11) of the larval 

 excretory organs of Nereis (Taf, xxvii, figs. 2, 3) and of Polymina 

 (Taf. xxvii, fig. 11) may be especially alluded to. The possibility 

 of the conversion of the internal end of a head-kidney-like nephridium 

 into a ciliated funnel, and of the entire nephridium into a vesicula 

 seminalis, is a fact (if it be a fact) of some morphological interest. 



Whilst the excretory nephridia of the male D. tseniatus open into 

 a space which has been described above as a part of the body-cavity, 

 the vesiculse seminales open into the cavity of the testis. In 

 certain other Archiannelids [Protodrilus , Polycprdius), the space 

 which is partially lined by generative cells is certainly part of the 

 body-cavifcy. From the analogy of these forms, it may perhaps be 

 concluded that, in Dinophilus, the hardly differentiated space which 

 occurs in the interior of the ripe testis is also a part of the body- 

 cavity. In this case we could assume that whilst the excretory 

 nephridia open into the general body-cavity, the vesiculse seminales 

 of D. tseniatus have acquired an opening into a special generative 

 division of the cavity. Attention may be called to the similarity 

 between the young generative organs shown in fig. 11 and the 

 mesoblastic bands of a Chaetopod larva, and also to the similarity 

 between the subsequent history of the testis of D. tseniatus and of 

 the body-cavity of the developing Chgetopod. Although I make 



