282 OTT. [Vol. VII. 



near the posterior end of this area (Figs, i and 13, op). It lies 

 on the ventral side of the body. 



The tube is formed at the posterior end of the body by the 

 union of very many fine branches which are scattered through 

 the dorsal part of the posterior end of the body. This tube, 

 while it gradually increases in size, passes forward in a very 

 tortuous course dorsal to the alimentary canal, and between the 

 lobes of the brain to the point midway between and anterior to 

 the ciliated pits. Here it makes a sharp bend toward the ven- 

 tral side and increasing gradually in diameter pursues a much 

 less tortuous course dorsal also to the alimentary canal to the 

 opening at the posterior end of the body. In some worms 

 the two limbs of this tube lie in the same horizontal plane 

 between the ciliated pits and the mouth, while behind the 

 mouth the larger limb lies ventral to the smaller. In other 

 cases the larger limb lies ventral to the smaller throughout 

 its whole course. The larger limb increases in size from the 

 anterior end of the body to the opening at the posterior end. 

 Thus there is a continual increase in the size of the tube from 

 its beginning to its external opening. Over the anterior end 

 of the pharynx the dorsal limb is 6 /* in diameter, and the 

 ventral is 7 /* in diameter. 



Numerous fine lateral branches, which are themselves made 

 up by the junction of smaller branches, join the dorsal limb of 

 this tube at intervals along its length. These branches are 

 more numerous at the posterior end of the body and gradually 

 decrease in number towards the anterior end. 



It is with great difficulty that these fine branches may be seen. 

 They are visible only in the smallest worms which have the 

 alimentary canal nearly or quite free from food material, and are 

 therefore nearly transparent. In favorable conditions in the liv- 

 ing worm ciliary motion may be seen in the posterior end of the 

 ventral limb of the tube, and also in both limbs near their 

 junction. 



In the longitudinal sections the ventral limb of the tube may 

 be traced from the anterior to the posterior end of the body. 

 In these sections, and also in cross-sections, the walls of this 

 tube may be seen to be made up of a single layer of cubical, 

 ciliated cells (Figs. 14 and 15). The length of their axes is 

 3.5 /A. These cells are finely granular and have very large. 



