No. 3.] THE DISCODRILID NEPHRIDIUM. 337 



In Bdellodrilus the nephrostomal lip is formed of only two 

 marginal cells (PI. XXI, Figs. 5, 7, 8 mn). These are not of 

 equal thickness all around, but each is conspicuously swollen 

 dorsally, and much thinner ventrally (Fig. 10^), thereby causing 

 the obliquity of the terminal face and the excentric position of 

 the nephrostome. In B. illuminatus the marginal cells are, 

 during life, conspicuously roughened by rounded eminences 

 which simulate cells and are richly ciliated. Branchiobdella 

 instabilia and pulcherrima have three marginal cells, two of 

 which are dorsal and swollen, the third ventral and much 

 thinner. This latter may really be slightly displaced from the 

 margin and correspond more nearly to the central celj of 

 Lumbricus, etc. Fig. 9 is a dorsal view. 



Longitudinal sections of the funnel of B. philadelphicus 

 (Fig. 10) show a direct continuity i^pni) of the outer boundaries 

 of the marginal cells with the unmodified peritoneal layer which 

 invests the remainder of the funnel and nephridium. This 

 layer (/) is not distinctly cellular, but if one of the large nuclei 

 which here and there occur (PI. XXIII, Fig. 52) be near for 

 comparison, as in the funnel figured, the resemblance to the 

 nuclei of the marginal cells is very striking. The latter have 

 the appearance and anatomical relations of greatly enlarged 

 peritoneal cells. During life the protoplasm of these cells is 

 transparent, with rather large and sparse granules. The nuclei 

 are very transparent, of large size, and with a single prominent 

 chromatin body. The whole outer and nephrostomal surfaces 

 are covered with rather short cilia (Figs. 5 et seq.), which lash 

 so vigorously that the funnel vibrates with the motion, as does 

 a Vorticella when feeding. In sections stained with haema- 

 toxylin (PI. XXI, Figs. 10, lo^ 10^) the protoplasm appears 

 very homogeneous, except in a zone superficial to the nucleus, 

 which is more granular and more intensely stained. 



The funnel proper is completed by a single additional cell, 

 which in its optical characters, both when living and when 

 stained, does not differ from the marginal cells. This I have 

 called the central cell, but it differs from the central cell of the 

 funnel of the earthworms (s) in being tubular. This cell (in 

 Figs. 6, 7, 8, 10, and 10'^) joins the marginal cells on the one 



