Illinois OligocJuefa. 447 



main mass at a point a little anterior to the middle and 

 extending to the body wall in a latero-ventral and posterior 

 direction, opening a little anterior to the inner margin of the 

 ventral bundle. Absence of nephridia from one side of the body 

 in several successive somites has been frequently observed 

 in Naidomorpha (Benham,'93, p. 385, and Smith, '96, p. 398) 

 and occasionally in the Tithificidfc (Stole, '88, p. 23). In 

 nearly all the species of THbificidce studied by this latter 

 observer he found rare cases in which one of the nephridia of 

 some somites was entirely degenerate, and others in which a 

 whole row of nephridia on one side, especially in the last 

 somites, was less developed than the row of the other side. In 

 Rhizodrilus lacteus this degeneration has gone still further, 

 so that a row of nephridia on one side is entirely wanting. 



The testes are in the anterior part of X, in the usual posi- 

 tion, and the spermiducal funnels are in the posterior part 

 of this somite, the sperm- ducts leading from them into XI 

 and opening on its ventral side. Following the sperm-duct 

 backwards from the point of entrance into XI, it is at first of 

 small diameter, with relatively thin walls, and extends pos- 

 teriorly and somewhat ventrally for a distance equal to one 

 sixth the length of the somite. Bending then abruptly toward 

 the dorsal side of the somite it passes directly upward to the 

 dorsal wall, and then extends in a posterior direction to a 

 point slightly beyond the middle of the somite, where it bends 

 toward the ventral side, opening on the antero-lateral wall of 

 a spermiducal chamber into which opens also the sperm- 

 duct of the other side. (PI. XXXIX., Fig. 5.) From the 

 point at which the sperm-duct bends towards the dorsal wall 

 throughout the rest of its course the lumen remains of small 

 diameter. From this same point on, for nearly its whole 

 length, the walls are greatly thickened, so that the diameter 

 of the duct is nearly one third that of the entire somite. A 

 short distance from the spermiducal pore the walls become 

 thin again. There is no enlargement of the lumen to form 

 an atrium, nor any modification of the wall in any special 

 region to form a prostate gland, nor is there a penis. The 

 great diameter of the duct throughout most of its length is 



