46 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 97 



end blindly, but may be branched ; each tube or each branch terminates 

 with a cell that sends a long vibratile flagellum into the lumen of the 

 tube. The larval protonephridia are apparently of ectodermal origin, 

 being said to be formed from primary nephroblasts derived from 

 cells of the third quartet of blastomeres ; their structure is essentially 

 that of the "flame cell" tubes of the excretory canals of the Platy- 

 helminthes. Since the larval nephridia are present before the coelomic 

 sacs are formed, they lie within the primary body cavity, which later 

 becomes the haemocoele. 



A type of closed nephridium resembling the larval nephridia, and 

 therefore often called a protonephridium, occurs in the five pregenital 

 somites of the archiannelid Dinophilus (fig. 13 A), and in the post- 

 larval somites of several families of Polychaeta. The closed nephridia 

 of the adult worm, however, are more highly developed excretory 

 structures than the larval organs, and usually have a more complex 

 end apparatus of tube-cells (solenocytes), which contain long fila- 

 ments resembling the flagella of the larval nephridia, but said to be 

 nonmotile. The nephridial canal has a simple structure, and its lumen 

 is ciliated. These nephridia project into the coelomic cavities, but, 

 inasmuch as they are ensheathed in folds of the peritoneum, they lie 

 morphologically in the haemocoele. Because of the similarity of their 

 structure to that of the larval nephridia, the closed nephridia of the 

 adult are supposed also to be of ectodermal origin, but their develop- 

 ment apparently has not been studied. 



The usual adult excretory organ, occurring in most Archiannelida 

 and Polychaeta, and in all Oligochaeta, is of the type called a mcta- 

 nephridimn. The characteristic feature of a metanephridium is the 

 presence of an inner opening, or nephrostome, by which the nephridial 

 canal communicates with the coelom. Solenocytes in this case are 

 absent. The nephrostome may be a simple ciliated aperture, as in 

 the archiannelids, but more commonly it has the form of a wide, open, 

 ciliated funnel. Unless coelomic dissepiments are absent, the nephro- 

 stome always lies in the anterior lamella of the dissepiment before 

 the somite in which the canal opens to the exterior. The canal thus 

 appears to traverse the coelomic cavity behind the funnel, but morpho- 

 logically it is extracoelomic, since it is ensheathed in a peritoneal fold 

 produced from the posterior lamella of the dissepiment bearing its 

 funnel. A closed nephridium is without doubt strictly an excretory 

 organ, but an open nephridium may serve both for the removal of 

 excretory products and for the discharge of the gametes from the 

 coelom. 



