342 MOORE. [Vol. XIII. 



sometimes at one end. As no nuclei are found in the plexus 

 nodules, it may be concluded that each of the drain-pipe cells 

 in this region includes a short simple tubule as its middle por- 

 tion, and part of a plexus nodule at each end. This, however, 

 will be referred to again. 



By careful observation of the ciliary action in the connecting 

 tubules of removed nephridia, remembering always that the 

 waves of movement travel toward the nephridio-pore, the tor- 

 tuous course of the tubule through the plexus region may be 

 traced. Here and there spots may be obscure, but if the 

 nephridium has been mounted in a normal fiuid,^ it will live 

 for a long time (one-half hour or more), and as the cover glass 

 gradually settles down, these points become clearer. Or better 

 yet, the tubule may be slightly unravelled by the careful use 

 of needles, and the whole course traced. Fig. i was drawn 

 from such a dissection, in which the crowded nodules were 

 gently pulled asunder and the tubule folds opened. After 

 drawing the living specimen, the whole was stained in methylene 

 blue to bring out the nuclei. Such stained specimens some- 

 times showed the plexuses very vividly, as the stain was drawn 

 into the passages by the ciliary action and their immediate 

 walls stained very quickly. 



Reference to Figs, i and 2 shows, regarding the arrangement 

 of this region : 



First: That the alternating plexus nodules and connecting 

 tubes constitute a continuous tubule, the lumen of which is in 

 every part distinct and enclosed in unbroken protoplasmic walls ; 

 and that there is nowhere any indication of any anastomoses 

 between contiguous portions of the tubule folds. 



Second : That in about seventeen of the nodules the plexuses 

 are well developed, and that in this region, making up the main 

 plexus lobe, the folds of the tubules are arranged along a por- 

 tion of the efferent duct as an axis. The exact arrangement is 

 not shown in Fig. i, in which the parts are somewhat displaced, 

 but the plan is indicated in the diagram. Fig. 2. Along the 

 axial tubule (ef^, et~) the plexus tubule bends more or less 



1 I used as most convenient at the time the blood fluid of the crayfish, diluted 

 with an equal volume of distilled water. 



