263] NORTH AMERICAN MONOSTOMES 45 



ior end. The excretory system as far as can be made out in preserved 

 material conforms in this species to the description given previously in 

 having a single thin walled vesicle in the extreme posterior end of the body 

 into which the anastomosing tubules empty. It opens to the exterior 

 slightly dorsal to the posterior end. The genital glands in general occupy 

 the intercecal zone and fill entirely that space. The posterior testis occupies 

 the intestinal arch, is usually spherical in form and measures 783 to 984/x 

 in diameter. The anterior testis is usually removed from the posterior by 

 several uterus loops. It is spherical and approximately the same size as 

 the posterior measuring 730 to 860ju in diameter. As in other species of 

 this genus the vasa efferentia unite cephalad and mesad to the anterior 

 testis. From this point the vas deferens takes its course among the uterine 

 folds to the posterior end of the cirrus pouch which is situated at the 

 middle of the intestinal bifurcation. The cirrus pouch extends from the 

 genital atrium caudad to the middle of the intestinal bifurcation. From 

 this point the club-shaped cirrus pouch extends cephalad to the genital 

 atrium and lies ventral to the anterior end of the pharynx. The ovarian 

 complex is situated between the testes and adjacent to the crura opposite 

 the anterior testis. The ovary is spherical and measures 413 to 463 n in 

 diameter. Dorsal and posterior to the ovary is the spherical receptaculum 

 seminis, 165ju in diameter. The duct of this unites with the oviduct 

 before it enters the shell gland. The shell gland is approximately the same 

 size as the ovary and is situated posterior and dorsal to that organ (Fig. 

 9). The well developed follicular yolk glands occupy the region of the 

 body lateral to the intestinal crura and extend from the anterior-most 

 part of the intestinal bifurcation to the posterior end where they are 

 separated only by the small excretory bladder. The follicles are arranged 

 in clusters on secondary branches from the main stem and in this manner 

 extend laterad around the crura both dorsally and ventrally, in many 

 cases reaching out as far as the inner wall of the crura. The vitelline ducts 

 are given off in the region of the shell gland and pass mesad to a point 

 just dorsal to the shell gland where they unite to form the vitelline reser- 

 voir. From this the common vitelline duct passes dorsal to the shell gland 

 and joins the oviduct just before its entrance into that organ. Immediately 

 upon entering the shell gland the oviduct enlarges to form the ootype. 

 Upon emergence from this the uterus enlarges to form the receptaculum 

 seminalis uterinum. From this point the much folded uterus fills out the 

 entire space between the crura and passes over the bifurcation in a relative- 

 ly straight stretch to the genital atrium. The numerous eggs which fill the 

 uterus are thick shelled ovals, measuring from 122 to 153/i in length by 56 

 to 66 n in width. In general the eggs are smaller in the beginning of the 

 uterus than they are near the genital orifice. The eggs in the anterior 

 region show well developed miracidia with double dark eye spots. 



