16 ILLINOIS BIOLOGICAL MONOGRAPHS [462 



scattered and continue as two longitudinal lines to the posterior 

 end of the body on each side of the excretory bladder (Fig. 5, 

 p. 1). These lines of pigment extend thru the whole dorsal ventral 

 thickness of the body and are as apparent on one side as on the other. 

 Along each edge and appearing ventrally are two other irregular lines 

 of pigment which do not extend as far back. Little pigment flecks 

 are scattered from these lines out thru the body. In the young cercaria 

 the pigment develops first around the eyespots in dense masses and 

 spreads gradually with growth both forward and backward along the 

 lines mentioned. It is only in the oldest cercaria that it is spread suffi- 

 ciently to form the anterior spot and the lines extending to the posterior 

 end. 



The digestive system (Fig. 5) of Cercaria urbanensis is like that 

 described for other monostome cercariae. The mouth at the anterior 

 tip is sub-ventral and the oral cavity is surrounded by a relatively small 

 oral sucker, averaging 0.043 mm. in length and 0.049 mm. in width. 

 The esophagus is very narrow and 0.05 mm. to 0.06 mm. long depending 

 on the contraction. The cecal bifurcations are close together at their 

 beginnings but soon spread further apart. They are always within and 

 slightly dorsad of the large longitudinal excretory vessels and extend 

 almost to the end of the body. The ceca are not yet functional, being 

 composed of a solid mass of cells. 



The excretory system (Fig. 5, ex) is typical of the group. At the 

 posterior end is the excretory bladder opening just below the base 

 of the tail. From this extends forward two large vessels which unite 

 in the midline just back of the oral sucker. Thruout their whole course 

 they are filled with small round concretions which disappear in the 

 process of preservation. It is very difficult to be positive of the rela- 

 tions of the excretory system of the tail but as nearly as they can be 

 made out they are as follows : No openings could be found in the 

 tail and a single vessel passed forward from near the tip becoming 

 larger nearer the body and opening at the excretory pore. 



Almost the whole body of Cercaria urbanensis is filled when mature 

 witli large unicellular cystogenous glands containing small granules. 

 Only the very anterior tip, the posterior locomotor projections and the 

 tail are free from them. 



Only a few monostome cercariae have been recognized. All of these 

 except Cercaria lophocera Filippi (1857:5) correspond very closely in 

 structure to Cercaria urbanensis. Cercaria imbricata Looss is distin- 

 guished by the fact that the rediae have smaller intestines and lateral 

 appendages and by the structure of the posterior locomotor projections 

 of the cercaria (Looss 1896:192-197). Several different forms have 



