48 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL.97 



On the contrary, nearly all other investigators have stoutly main- 

 tained that both the funnels and the canals are mesodermal, though 

 some regard these two parts as derived from separate rudiments. In 

 this class may be mentioned E. Meyer (1887, Psygmobranchus), 

 Bergh (1888, Criodrilus, 1890, Lumhricus, 1899, Rhynchelmis) , 

 Burger (1891, Nephelis, 1894, Hirudo, Aulastomum) , Michel (1898, 

 Allolohophora), Lillie (1906, Arenicola), Bychowsky (1921, Clep- 

 sine), Penners (1924, Tuhifex), and A. Meyer (1929, Tnhifex). 

 Only Bergh is insistent that the entire nephridium is mesodermal ; 

 most of the others admit that a terminal part, perhaps including the 

 reservoir, may be formed from the ectoderm. 



According to Lillie, the nephridia of the polychaete Arenicola 

 cristata are gradually differentiated in the somatic mesoderm, starting 

 from the posterior angles between the septa and the body wall, but 

 the mesoderm in early stages of somite formation presents no cell 

 boundaries. The lumen of each organ appears as a minute intra- 

 cellular canal, which from its inception opens through the dissepi- 

 ment into the preceding coelomic cavity. Later, as the nephridial 

 cells divide, the lumen becomes intercellular, and finally it opens 

 posteriorly through the ectoderm. Lillie says, however, that there is 

 no invagination of the ectoderm, and no specific evidence that the 

 reservoir is an ectodermal formation. 



Those writers who claim that the nephridia of the Oligochaeta 

 and Hirudinea are of mesodermal origin agree essentially with Bergh 

 that each organ is formed from a single cell of the anterior lamella 

 of an intersegmental septum. According to A. Meyer (1929), for 

 example, the nephridioblasts of Tuhifex are early differentiated from 

 the other cells of the septa by their large size (fig. 20 A, Nphl). By 

 successive divisions of the nephridioblast a column of cells is formed 

 that pushes backward within a sheath of ordinary epithelial cells 

 derived from the posterior lamella of the septum (B-E). The young 

 nephridium extends in a space between the somatopleure and the 

 longitudinal muscles, and is thus extracoelomic. The lumen appears 

 first as an intracellular canal, which later becomes intercellular by a 

 radial division of the cells; it is ciliated from an early stage. Pos- 

 teriorly the canal ends against an epidermal cell (G), through which 

 it eventually opens to the exterior, and from which is later gener- 

 ated the reservoir. The coelomic funnel is formed by the original 

 teloblast, the nucleus of which divides into four nuclei, one taking 

 a position in the dorsal lip of the funnel, the other three in the ventral 

 lip (H, I). According to Bergh (1899) only the lower lip of the 

 funnel in Rhynchelmis is derived from the nephridioblast, the upper 



