497] LARVAL TREMATODES—CORT 51 



large in his form as in Cercaria douthitti. The acetabulum is fully func- 

 tional as noted above. It is very small, on the average 0.025 mm. in dia- 

 meter, situated just back of the middle of the body. 



The eye-spots are in front of the middle of the body and located 

 nearer the dorsal side. The pigment masses which compose them are 

 in the shape of concavo-convex discs, 0.007 mm. in diameter, placed so 

 that the concave surfaces are toward the sides of the body. Fitting into 

 the concave side of each is a small lens. In the region of the eyes and 

 lying just ventrad and behind them is the central nervous system. A 

 cross section of the cercaria in the region of the eye-spots brings out 

 these relations clearly (Fig. 56, e). 



The region back of the center of the body of Cercaria douthitti 

 is almost completely filled with large unicellular cephalic glands, the 

 usual number of which is eight. The most anterior reach to the middle 

 of the body and from all, large ducts extend forward in two groups. 

 These two groups of ducts pass thru the sheath of the oral sucker pos- 

 teriorly and traverse this organ to open at the anterior tip of the body 

 (Fig. 59, 60, 61, 62). No cephalic spine is present altho these glands 

 appear to be analogous to stylet glands. Cercaria ocellata La Valette St. 

 George (1855:22-23) is the only other furcocercous cercaria in which 

 such glands are described. The glands are flasked-shaped and have a 

 length varying from 0.04 mm to 0.05 mm. and a width of from 0.025 

 to 0.03 mm. The thickness is about equal to the width. A traverse 

 section (Fig. 58) thru the acetabular region of Cercaria douthitti shows 

 how much of the body space is taken up by the cephalic glands. 



At the posterior end of the body of Cercaria douthitti is a small 

 excretory vesicle from which two crura could be traced forward only as 

 far as the acetabulum. Backward from the bladder two small vessels 

 pass into the tail (Fig. 57, ex). These soon unite into the central caudal 

 vessel which divides to run down the lobes and opens at their tips (Fig. 

 55). No concretions of any kind were present in the excretory system. 



Just in front of the excretory bladder and wedged between the tips 

 of the cephalic glands, is a small mass of nuclei the anlagen of the repro- 

 ductive organs. 



The furcocercous or forked-tailed cercariae are very imperfectly 

 known. The anatomy of only a few of the known forms is at all well 

 worked out and the life-history of no one of them has been determined. 

 At least a dozen species have been reported as distinct, some of which, 

 however, have been described very briefly in the older accounts. Suffi- 

 cient evidence is not available to justify any conclusion as to the natural 

 or artificial character of this group. 



