348 MOORE. [Vol. XIII. 



more internal one, and it is this one which is most frequently 

 developed, a group of long lash-like cilia are attached. These 

 are placed in a more or less sharply defined longitudinal tract, 

 and are like those of the connecting tubules of the plexus 

 region. A second diaphragm may be developed opposite to 

 the point reached by the free ends of the cilia, but this is 

 always less prominent than the other, is usually very faintly 

 visible, and is very often wanting. Occasionally it will bear a 

 group of smaller cilia, but this is a rare condition. Sometimes 

 the longer cilia will project through the perforation in the 

 second diaphragm, and their tips may be seen beating spirally 

 on the far side. Usually they are confined within the chamber 

 between the two diaphragms when these are present. These 

 peculiar structures probably act as valves, though they would 

 seem to permit the flow of fluids either way with equal facility. 

 They may, as well as the enlargements of the lumen in which 

 the cilia lie, be simply the mechanical result of the activity of 

 the latter. 



In the corresponding region of the post-genital nephridia the 

 tubules have a diameter of only .0125 mm., and their lumen of 

 .009 mm. Their structure, however, does not differ, but the 

 arrangement is more irregular, and the two limbs of a loop are 

 often independent for considerable distances. 



The shorter tubule loop is confined to the large lobe, in which 

 it extends from the base not quite to the end of the longer loop, 

 and, as it is nearly straight, is considerably shorter than the 

 latter. Its outer limb is connected with the inner limb of the 

 long loop by a short connecting tubule (Fig. i, cct), which lies 

 in the small lobe in contact with the accessory plexus lobe, to 

 the irregularities of which it conforms, and around the most 

 internal nodule of which it passes before joining the short loop. 

 In its structure this section of the tubule shows transitional 

 characters, in that its internal end resembles the tubule of the 

 long loop, its external that of the short loop. It possesses 

 never more than one nucleus and one group of cilia, the posi- 

 tion of which is shown in Fig. i. The inner limb of the 

 shorter loop crosses the outer at the base of the large lobe 

 and passes into the efferent connecting duct (Fig. i, ef^). 



