58 ILUNOIS BIOLOGICAL MONOGRAPHS [58 



example of this valueless type of description is afforded in the form Diplosto- 

 mulum cuticula (von Nordmann 1832), reported by four American investigators 

 from various localities east of the Rocky Moimtains. The descriptions include 

 larvae encysted with pigment and without it, some specimens found subder- 

 mally, others taken from the peritoneum of the body cavity, all secured from 

 a great variety of teleost fishes. In none of the descriptions is there mention 

 of the course of the excretory system or of the genital cell masses, both of which 

 are essential to the exact determination of the species. It seems reasonably 

 certain that a careful revision of these forms described as Diplostomulum 

 cuticula (von Nordmann) would result in the discovery of several new species 

 of Diplostomulvun. 



The species of cercariae and parthenitae described in this section of the 

 paper have been studied with special reference to the excretory, genital, and 

 nervous systems. 



MONOSTOMATA 



Cercaria pellucida Faust 1917 



This larval trematode is a muscular cercaria, characterized by heavy 

 anterior pigmentation on the dorsal surface, centered around three foci, the 

 paired lateral eye-spots and the median eye. It is a member of the trioculate 

 group of the Monostomata. The pigmentation tends to spread caudad from 

 the pigment center along six Hues of growth, two dorsal, two lateral and two 

 ventral. These lines of pigment have been shown (p. 53) to be the superficial 

 index of the underlying nerve trunks. The worm is characterized by 1) a 

 transparent body, 2) a circuit of refractory granules that marks the excretory 

 system, 3) a small oral sucker, and 4) large longitudinal muscle bimdles of 

 the tail. 



Cercaria pellucida was obtained from Lymnaea proxima Lea in the Bitter 

 Root River in the vicinity of Corvallis, Montana, and from Physa gyrina Say 

 near Buckhouse Bridge. The snails were examined in October, 1916. 

 Lymnaea proxima contained a heavy infection with this species, along with a 

 lesser infection with the monostome cercaria, C. konadensis, and a distome 

 larva, C. diaphana. Physa was heavily infected with an echinostome, C. 

 trisolenata, and contained only a hght infection with the monostome. In all 

 cases the infected organs were the Uver ceca. 



The mature C. pellucida has an average measurement of 0.4 mm. to 

 0.7 mm. in length and 0.18 mm. to 0.2 mm. in width. The tail is about 0.5 

 mm. long and has a diameter of 0.07 mm. at the base. Most usually the 

 animal has an eUiptical constricted outline, such as is shown in figiu-e 4, but 

 when relaxed it assumes the elongate-ovoid or spatulate condition, shown in 

 figures 1 to 3. 



The parthenita is a large conspicuous redia, measuring 2.2 mm. by 0.5 mm. 

 (Fig. 6). Within the redia is a large rhabdocoel gut extending almost the 

 entire length of the animal and measuring 0.3 mm. in cross section. The gut 

 empties anteriad thru a muscular bulbus 15m in length and 12/x in cross sec- 



