Premnodrilus palustris. 465 



about 1 mm. in length, of which the diameter is .015 to 

 .025 mm., while that of its lumen is .005 to .010 mm. The 

 walls of this duct include an inner epithelium, layers of 

 circular and longitudinal muscles, and an outer layer of 

 small, more or less isolated cells, like that already described 

 for the sperm-reservoir. This duct is usually contorted in 

 its course, and passes through the muscular Avail of the 

 inner end of the penial apparatus and opens into the inner 

 end of the greatly elongated penis. There is no " prostate" 

 nor "atrium" as in Eclipidrilus frigidus, and the muscular 

 duct, which is perhaps comparable to the "bridge" of 

 E. frigidus, connects the sperm-reservoir directly with the 

 penis. 



The proportions and relations of the penis in Premnodrilus 

 palustris can best be understood by reference to Fig. 1, 

 PI. XL I. The penial apparatus includes an elongated sac 

 lined with epithelium, continuous with the outer epithelial 

 layer of the penis proper, and, adjacent to this, a thick layer 

 of longitudinal muscle, outside of which is a delicate epithe- 

 lium. No layer of circular fibers is developed. In a speci- 

 men in which the penis is not much protruded the penial sac 

 extends upward and backward into the anterior part of XII, 

 the posterior septa of IX, X, and XI being forced back with 

 it. Strong muscular bands connect the free end of the sac 

 with the dorsal body wall of XII-XV, the strongest band 

 being in the posterior part of XIV. The long slender penis 

 is inclosed by the walls of the muscular sac, but is nowhere 

 connected with it except at its inner end. Its entire length 

 is 1.25 mm., of which .4 mm. is protruded from the body in 

 one of the specimens studied. Its diameter varies from .015 

 mm., near the tip, to .08 mm., near the point of attachment, 

 and at the middle is about .04 mm. Its unusual length is 

 doubtless correlated with the correspondingly long sperma- 

 thecal duct, to be described late,)-. 



The lumen of that portion of the sperm-duct which forms the 

 penis has a nearly straight course and is of small diameter, 

 being but .005 mm. at a point midway of its length. The 

 penis is covered superficially by an epithelial layer continuous 



