368 Ealph S. Lillie 



dorsal processes, the epithelium is folded (Text-fig-ure on p. 362) 

 and is composed of cells of a more columnar shape than tbose 

 covering the ventral lip, which are more cubical in appearance 

 (Piate 22, Fig. 6). The difterenee in shape is, liowever, inessential 

 and is due in all probability to the different conditions of pressure 

 and tension to which the cells are exptised in the ditlereut regions; 

 the epithelial cells of the processes — being apparently subjected 

 to lateral pressure, as indicated bv the folding — are for this reason, 

 more columnar; while the cells of the smooth unfolded epithelium 

 of the ventral vesicle are apparently subjected to the tangential 

 tension characteristic of distended vesicles, and are thus forced to 

 assume a more cubical shape. In all essential respects, liowever, 

 the cells are identical in structure. They are large and clear, of 

 approximately the same size in different regions, and do not contain 

 concretions. Each is bordered by numerous strong cilia (each of 

 which has a well-defined deeply staining basal granulo), and the 

 protoplasm exhibits in staiued preparations the fibrillar structure 

 characteristic of ciliated cells. As Gamble & Ashworth have 

 poiuted out (1900, p. 515) the cells are mudi larger than those of 

 the cubical epithelium of the early nephrostomes. The relative size 

 of the two may be seen by comparing Piate 22, Fig. 6 and Piate 25, 

 Fig. 47, which are drawn to the same scale. It will be Seen, 

 however, that the nuclei of the early nephrostome are of almost 

 exactl}' the same size as the nuclei of the adult structure, and occupy 

 a correspondingly large portion of the cell. In tlieir ciliation, 

 character of the protoplasm, and general shape, however, the cells 

 are closely sirailar. 



The cubical epithelium of the funnel passes over into the excre- 

 tory epithelium of the glandulär region by a somewhat abrupt 

 transition, although a few of the cubical cells immediately adjoining 

 the excretory epitliclium proper may contain granulös in their interior 

 and in other respects exhibit a partially intermediate structure. 



The large excretory cells of the glandulär portion of the 

 nephridium form a single layered excretory epithelium lining tbe 

 interior of the organ and generally thrown into a well marked system 

 of internal folds. External to the ei)ithelium is a thin connective 

 tissue layer hearing numerous blood-vessek; externally the whole 

 organ is covered by the Peritoneum. 



In structure the cells agree closely with those of Ä. ecaudata 

 as dcscribcd bv Gamble & Ashworth. Like most cells of an 



