18 ILLINOIS BIOLOGICAL MONOGRAPHS [464 



was infected with this same form. The second of these species, a 

 smaller unpigmented cercaria, was found in one of twenty specimens 

 of Planorbis trivolvis from a small pond in the suburbs of Chicago. 

 In all the infected snails mature and immature cercariae were found 

 free in the liver, the mature forms being nearest the periphery, and 

 the active rediae contained no fully developed cercariae. There were 

 no sporocysts present and no rediae in which rediae were develop- 

 ing, and in none of the infected snails were rediae or cercariae numerous. 

 Since the large pigmented species is very unwieldy in movement, I 

 propose to name it Cercaria inhabilis, and the smaller species on account 

 of the way in which it changes its body shape will be given the name, 

 Cercaria diastropha. 



Cercaria inhabilis swam sluggishly in open water. It contracted 

 its body and lashed its tail backward and forward, moving in an 

 unwieldy, irregular fashion. In fact the body was too large in propor- 

 tion to the size of the tail for rapid locomotion. On a substratum 

 the cercaria extended and contracted the body but was unable to move 

 by the aid of its suckers. 



When in a state of average contraction, about that of figure 16, 

 the body of Cercaria inhabilis is pear shaped, tapering in the anterior 

 half, and wider but of uniform diameter posteriorly. It is the largest 

 of the cercariae studied, having an average length in mounted speci- 

 mens of 0.8 mm. and a width of 0.4 mm. The thickness is a little 

 greater than half the width. The oral sucker is elongate, 0.16 mm. in 

 length and 0.12 mm. in width with the retrodorsal pharyngeal pockets 

 which are characteristic of some amphistomes. The acetabulum is very 

 large averaging 0.23 mm. in diameter; it is at the posterior end of 

 the body and is turned ventrad. 



Two large eye-spots are present just back of the pharynx. They 

 are located from one-fourth to one-third of the distance from the anterior 

 to the posterior end, and in a specimen 0.27 mm. wide at this region, 

 they were 0.065 mm. from the outer margins and 0.13 mm. apart. These 

 eyes are composed of the lens and the cone of pigment like those 

 already described for the monostome, Cercaria urbanensis. Figure 17, 

 a section thru the eyes, shows them in their relation to the nervous 

 system and other adjacent structures. 



In the development of Cercaria inhabilis the pigment starts in the 

 eyes and is deposited first in a peculiar way over most of the anterior 

 half of the body. In the youngest cercariae found outside of the 

 rediae very little pigment is seen and that found near the eyes 

 (Fig. 18, p.). In forms a little older (Fig. 19, p.), the pigment has 



