77] LIFE HISTOR Y OF TREMA TODES—FA UST 77 



upper and lower reaches of the Valley, and in Planorbis trivolvis from the region 

 of Buckhouse Bridge. The infection of the host is always heavy, both 

 as relates to numbers of individuals infected and the number of parasites in 

 the individual host. The per cent of infection ranges from 22 to 100. The 

 parasite is located primarily in the interstices of the liver, but frequently 

 invades the cecal walls and does great injury to the tissues. 



The cercaria develops in a redia of well-marked characters (Fig. 117). 

 The redia measures about 1.0 mm. in length and has a mid-diameter of 0.22 

 mm. and a gross width of 0.35 mm. across the region of the locomotor feet. 

 A small powerful pharynx at the anterior end opens into the rhabdocoel gut 

 which fills the greater part of the body cavity, extending almost to the posterior 

 extremity. The germinal epithehum is at the posterior end. From this 

 layer the ova develop, which grow into cercariae. The first character of the 

 germ ball to become differentiated superficially is the oral sucker (see series 

 of stages in figure 114). Later the tail and the acetabulum become marked 

 off, and finally the oral hood. 



At the posterior end a small, non-muscular, truncate bladder is situated. 

 It opens anteriad into two simple unbranched tubes. These can be traced 

 cephalad inside the intestinal ceca to the head of the worm. The cephalic 

 end of the excretor}- system is unique. Lateral to the pharynx, a triangular 

 channel-system is found. From the anterior angle a small capillary leads 

 forward to a single flame cell. From each of the other two angles a small 

 capillary leads back to a flame cell. This constitutes the trisolenate sys- 

 tem at the anterior terminus of the excretory tract. The tail excretory tube 

 is a single median structure and has no laterals or terminal outlet. This 

 fact necessitates a revision of the scheme proposed by Cort (1915:37), in 

 which this writer characterizes the excretory system of echinostome cercariae 

 as "opening on each side of the anterior part of the tail. " It seems from the 

 present investigation that the three flame cells in the anterior part of the 

 trunk may be a more reasonable criterion for distinguishing the excretory 

 system of this group. Further work on other forms must be done before this 

 can be definitely proposed. 



Excretory granules fill the lateral excretory trunks from the pharynx 

 region as far caudad as the acetabulum. 



The digestive system is simple. It consists of a long esophagus, with a 

 very small pharynx sphincter about in its middle, and two very long furcae, 

 extending to the sub-caudal region. 



The genital cell masses are yet \ery immature. There are four cell masses 

 present, one on the upper right of the acetabulum (Fig. 130), the vagina; one 

 behind the acetabulum (Fig. 131), the ovary, and two tandem masses in front 

 of the bladder (Fig. 120), the testes. In the vicinity of the ovary are num- 

 erous vitelline follicles, but they have not been found to follow any definite 

 pattern. 



