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EXCRETORY ORGANS OF METAZOA. 549 



Acoela. Three chief types have been distinguished (v. Graff, 1882) : 

 a single main canal with a single nephridiopore (Stenostoma) ; a 

 pair of main canals with independent nephridiopores, and a pair 

 of main canals with a common nephridiopore. In Bothrio plana 

 (Vejdovsky, 1895) there are two pores different in structure, one 

 at the middle and the other at the anterior end of the body; into 

 the former open two main canals, each of the latter divides into an 

 anterior and a posterior branch, and these anterior branches con- 

 nect also with the anterior nephridiopore. In the Eumesostomina 

 (Luther, 1904) the main canals open independently either on the 

 surface of the body, or into the mouth, or into the genital atrium 

 (all these being ectoblastic), and besides the terminal flame cells 

 there are other flames (without nuclei) interpolated in the course 

 of the canals. In these forms there is never more than one pair 

 of main canals, or more than two nephridopores. 



Tricladidea. — In Planaria but more specially Gxinda Lang 

 (1881) described two main ducts on each side of the body, each 

 bearing numerous capillaries ending in flame cells ; there are anasto- 

 moses between the former but not between the latter ; from each 

 main duct proceeds a series of excretory ducts each of which opens 

 dorsally by a small contractile vesicle. And Bohmig (1906) adds 

 to this account of Gunda by the discovery of four pairs of main 

 canals, and of ventral as well as dorsal nephridiopores. In Dendro- 

 ccclum Ijima (1885) found similar relations, though he held there 

 to be but one main canal on each side; while Wilhelmi (1906) 

 found two of them with a segmental arrangement of eight pairs 

 of excretory ducts, and (in opposition to the earlier observations 

 of Chichkoff) no openings into the pharynx. The Tricladidea 

 differ from the Rhabdocoelida in the presence of numerous serial 

 excretory ducts. 



(2) Treinatoda. 

 The chief characteristic of the excretory organs is their dendritic 

 branching and their degree of anastomosis. In the Monogenea 

 there are usually paired nephridiopores (in Gyrodactylus a single 

 one) placed in most cases at the anterior end but sometimes at the 

 posterior. The excretory vesicle of the Digenea is at the posterior 



