358 MOORE. [Vol. XIII. 



half are on the plexus mass. These are very conspicuous and 

 remarkably constant in position ; Fig. 1 5 shows one at p. 



In B. instabilia a distinction is noticeable between the plexal 

 and tubule peritoneum, the latter being thicker and perhaps 

 more granular, and with an occasional outstanding cell. The 

 nuclei are closer together, and at least twice as numerous as in 

 the last species. 



Bdellodrilus illuminatus differs again remarkably in the char- 

 acter of its nephridial epithelium. On the plexus region there 

 is nothing noteworthy, and in young examples (just hatched) 

 this is true of the tubule region also. As the worms increase 

 in size the number of cells investing the latter region becomes 

 greater, and at the same time they begin to grow out promi- 

 nently, first at the proximal end, and finally along the whole 

 length of the tubule lobe. The cells are at first rounded and 

 bulging, but finally elongate and stand out prominently from 

 the sides of the tubules. In the mature worm (Fig. i, /) 

 many of them are greatly elongated, with thickened basal ends 

 which fuse to form a common layer covering the tubules, but 

 remarkably thin and flat leaf -like distal ends which often branch. 

 The largest of these cells are grouped together principally at 

 two points, namely, at the distal end of the large lobe and at 

 its middle (Fig. i). Here their free ends are attached to the 

 body-walls, and those of the terminal group are partly arranged 

 side by side (but not coalesced) as a transverse sheet, uniting by 

 their free ends partly with the adjacent body-wall, partly with 

 the heart, and possibly one or two with the oesophagus. These 

 cells anchor the nephridium in place, and at the same time pre- 

 sent a great surface for excretion. They are easily detached, 

 however, and the torn ends present a frayed and ragged appear- 

 ance (Fig. i). Many of the cells never acquire these connec- 

 tions, and may be seen washing back and forth with the currents 

 in the coelomic fluid. Dark lines, mostly longitudinal and wavy, 

 mark the surfaces of the flattened portions of these very pellucid 

 cells, and are probably wrinkles, like those on a piece of crape 

 cloth. In addition the protoplasm is filled with numerous gran- 

 ular excretions, which in the full-grown worm are very minute, 

 very abundant, and gathered into little clusters (Fig. 53). 



