346 MOORE. [Vol. XIII. 



In B. instabilia, owing to the colorless blood, greater trans- 

 parency of the body-walls, and the thinness of their peritoneal 

 investments, the minute characters of the tubules are more 

 readily studied in the living condition than in B. illuminatus. 



In all of the species the two tubules lie in close contact, only 

 separating slightly here and there, where the spaces between 

 become filled by nests of connective-tissue cells (Fig. i, ctfi). 

 While in most parts the adjacent walls of the two tubules 

 remain distinct, they frequently fuse for a short distance, form- 

 ing a simple protoplasmic septum of variable thickness between 

 the two lumina (Fig. i). These septa are, however, never 

 perforated to permit of communication between the lumena on 

 either side, the only communication being the primary one at 

 the distal end of the loop, resulting from the simple continuity 

 of the tubule. Moreover, the lumina never encroach upon the 

 protoplasm of the conjoined tubule, and consequently we never 

 find here the condition of a single cell with a double lumen, but 

 only such a condition as would result if two drain-pipe tiles 

 were placed side by side in the plastic condition of manufacture 

 and pressed together until the line of junction was obliterated 

 by the union of the c'ay walls; a coat of cement would then 

 represent the investments. That the integrity of each tubule 

 is maintained throughout is further shown by the distribution 

 of the nuclei, which is such as to enable one always to refer 

 them decisively to one tubule or the other. In the longer 

 tubule loop of the anterior nephridium of B. illuminatus — and 

 the arrangement seems to be the same in other species — are 

 found twelve nuclei (Fig. i) distributed at fairly regular inter- 

 vals (the length of which, owing to the foldings of the tubules, 

 cannot be absolutely determined) of about .18 mm. The posi- 

 tions of these, as well as the windings of the tubules, are shown 

 in Fig. I, and are remarkably constant in a large number of 

 individuals. Each limb of the loop may be seen to contain one 

 nucleus of each of the six pairs in which they are disposed. 

 They lie in the protoplasmic wall close to the lumen and on 

 one or the other side; but whether always on morphologically 

 the same side or regularly alternating cannot, owing to the 

 twisting of the tubule, be confidently stated. 



