NOTES ON AOTINOTROOHA. 479 



been already said about the posterior nephridia. The 

 Actinotrocha studied by Masterman seems worthy of rein- 

 vestigatioD, if for no other reason than to demonstrate the 

 proboscis pores and the posterior nephridia. Tliere are two 

 nephridia in Actinotrocha. They are placed on the mesentery 

 between the collar and trunk cavities, on the face of it turned 

 towards the collar cavity. Each nephridium opens to the 

 exterior ventro-laterally behind the attachment of the 

 mesentery. From its external opening the duct of the 

 nephridium runs forwards, crosses the mesentery^ and then 

 bends dorsally and runs forwards along the mesentery. The 

 nephridium opens at its anterior end into the collar cavity by 

 two funnels, one placed below the other (fig. 8). Numerous 

 ovoid cells are attached by long stalks to the edges of the 

 funnels. I have not been able to detect cilia in the lumen of 

 the nephridium. The coilomic epithelium covers the outer 

 surface of the nephridium. The appearance of the nephri- 

 dium as seen in the living larva is so well described in the 

 quotation which Masterman gives that it is unnecessary to 

 say anything more about it. It is probable that this type of 

 nephridium, which so strongly recalls the nephridium of 

 Amphioxus, was characteristic of the primitive Chordata. 

 Masterman compares these nephridia to the collar canals of 

 Balanoglossus. There is no doubt a general resemblance 

 between them, and the collar canals may be vestiges of such 

 nephridia; but the detailed structure differs in the two cases. 

 It may also be pointed out that if the small part of the 

 nephridium which lies behind the mesentery were better 

 developed, as it is in the adult Phorouis, the nephridium 

 would be considered as belonging to the trunk, with its 

 funnels opening into the cavity of the segment in front. 



These nepliridia become the nephridia of the adult Phoronis. 

 In the newly metamorphosed Phoronis the nephridia still 

 project into the ii'regular collar spaces, but their external 

 apertures have shifted dorso-laterally, owing, no doubt, to the 

 mechanical narrowing of the dorsal surface of the body 

 during metamorphosis. 



