468 Scientific Proceedings, Royal Dublin Society. 



of the ccelom, and retains its connexion with the exterior as an 

 abdominal pore." 



Lang (7) appears to have been the first to compare the pores 

 which put the gastro- vascular system of Ooelenterates into direct 

 communication with the exterior with structures found outside that 

 group. He says : — " In certain Polyclades [Turbellaria] ramifi- 

 cations of the intestine open to the exterior by excretory pores, 

 either on the dorsal surface (Planaria aurantiaca d. Ch.), or on 

 the lateral edge (as in a very interesting new genus of the family 

 of Proceridse), thus forming a complete analogy with the ex- 

 cretory pores which are found at the edge of the bell in certain 

 Medusse. 



" The aquiferous system characteristic of other Platyelminths 

 does not occur in the Polyclades. The secretory organs of these 

 animals are formed after the type of those of the Ooelenterata . 

 excretion in the two groups is performed by means of diverticula 

 from the intestine which open to the exterior." 



Van Wijhe (13) believes that "the primitive Craniotes pos- 

 sessed no pronephric duct, the pronephros opening to the exterior 

 by a pore laterally from the gland. This orifice migrated later 

 posteriorly, and its outer border developed into the duct, and 

 coming into contact with the cloaca, opened into it." He further 

 goes on to say, that the epiblastic origin of the segmental duct 

 will not be welcome to those who hold that the Chordata were 

 descended from Annelids ; but, for his part, he cannot admit the 

 relationship between these types. 



"Without at all committing myself to a belief in the ancestry of 

 the Chordata from Chsetopod Worms, I would offer the following 

 considerations as tending to show that the Yertebrate excretory 

 system is readily comparable with that of Annelids, now that the 

 epiblastic origin of the segmental duct has been established. 



It is perfectly well known that the nephridia of all Inverte- 

 brates open directly to the exterior, and in the segmented Worms 

 there are typically a pair of nephridia for each somite. The 

 diagrams (Plate X., figs. 1 and 2) schematically represent this 

 arrangement. 



It is generally admitted that the early (not necessarily the- 

 primitive) Chordata were segmented, and it is not unreasonable to 

 suppose that the nephridia were segmentally disposed, as there is. 



