81] LIFE HISTORY OF TREMATODES— FAUST 81 



feature of this cercaria, in common with that of C. douthitti, is the possession 

 of eye-spots (Figs. 144, 150). But the eye-spots of C. gracillima are the more 

 vestigial, for they have no pigment. 



This species was found in the livers of Fhysa gyrina Say, collected from 

 the lower reaches of the Bitter Root River near Maclay Bridge, Buckhouse 

 Bridge, and the sloughs at the Roadhouse, near Fort Missoula, in the fall of 

 1916. In addition it was found in the livers of Lymnaea proxima Lea from 

 Rattlesnake Creek, Missoula. The infection in most cases was not exceed- 

 ingly heavy, except in the collection from the sloughs at the Roadhouse, 

 where thirty-three out of seventy-one individuals were infected, or 46.5 per 

 cent. 



The cercariae develop in long cylindrical sporocysts, varying in length 

 from 0.25 mm. to 0.1 mm., but most frequently averaging about 0.5 mm. 

 (Figs. 146, 147). In diameter the sporocysts vary from 0.2 mm. to 0.4 mm. 

 The parthenita is a simple structure, non-muscular, depending on the daughter 

 cercariae for its movement. At one end (Fig. 147) is a non-muscular attach- 

 ment area; at the other end, merely a rounded non-differentiated cap. The 

 germinal epithehum is localized at the attachment end. From this mass the 

 daughter worms develop. Internal pressure from the developing larvae 

 increases the length and diameter of the parthenita. In development (Fig, 

 148), the ovoid germ ball first differentiates a tail portion; later the rami 

 appear. It is not until considerably later that the acetabulum is found. The 

 oral spines appear only when the larva is mature. 



The movement of the cercaria is characteristic for furcocercariae. The 

 main movement consists in a very strenuous beating and lashing of the rami, 

 so that the head is pushed into the object with which it comes in contact. 

 In case the head is not forced into the object, the worm is set free by a back- 

 ward movement of the tail and the entire worm squirms around until it comes 

 in contact with another object, when the same boring movement is again 

 attempted. The oral end of the cercaria is much better adapted to this type 

 of invasion of the tissues to be infected than if it possessed an oral disc. 



The excretory system of C. gracillima is embryologically a single paired 

 system for both body and tail. As development and differentiation of parts 

 progress the tubes in the posterior extremity of the trunk and the anterior 

 region of the tail fuse, to form a median bladder and the common tube of the 

 proximal region of the tail. There remains the bifurcated portion in the 

 laterals of the trunk and the rami of the tail, and in addition, the "eyelet 

 anastomosis." This eyelet structure has been observed by Looss (1896:172- 

 174; PL 15), m Cercaria vivax Sons, and by Cort (1915, Fig. 57) in C. douthitti. 



The excretory system in the body consists of two lateral tubes that diverge 

 from the bladder and can be traced forward, together with dendritic tubules 

 and capillaries, the internal ones of which frequently form chiasmic anasto- 

 moses across the median plane of the body. Slightly posterior to the middle 

 of the body the lateral tube expands and opens into a pocket provided with 



