4S7 



Studies on the Development of Larvai Nephridia. 



Part I. Phoronis. 

 By 



Cresswell Shearer, 



Trinity College Cambridge. 



With Platea 31—33. 



General introduction. 



Some twenty years ago it was pointed out by R. S. Bergh (1) 

 tliat the provisionai larvai nephridia of some Oligochaets, Mollusca 

 and the head-kidney of the Polygordius larva, were probably one 

 and the same structure, and homologous with the excretory system 

 of Platyhelminths ^ It consists, in these animals, of a simple or 

 branched tube openiug on the exterior, internally being closed by 

 one or many peculiarly modified fiame-cells. It lies not in the true 

 coelom, biit in the Spaces of the mesodermie tissue or blastocoel 

 outside the coelom. Since the adult nephridia, in distinction to the 

 provisionai nephridia, ha ve alvrays some relationship with the coelom, 

 Heugh considered the two sets of org-ans distinct. The adult 

 nephridia were to be homologised with the ducts of the germinai 

 follicles of a Nemertine, while the cavities of these follicles them- 

 selves, were to be compared with the coelom of Annelids. It was 

 soon pointed out, that Bergh's objection to the homolog v between 

 , larvai and permanent nephridia, on the ground that the former do 

 Inot lie in the",true coelom, < does ^not hold, when we consider that 

 iü the forms that Bergh studied, the Gnathobdellidae, the provisionai 



' I am aware that Beugh was by no means the first to express this 

 liomology of the larvai excretory organa, in fact the whole of Bergh's theory 

 was somewhat tbrestalled in the well known words of Hatsciiek, published 

 äome timo previous: »Die secundäre Leibeshöhle verhält sich wie die Höhle 

 1er Geschlechtsdrüse der niedrigeren Formen.« Stud. ii. Entwickel. der Anne- 

 iden. in: Arb. Z. Inst. Wien 3. Bd. !>>Th p. yu . 



Ü 



