22 ILLINOIS BIOLOGICAL MONOGRAPHS [468 



Since Cercaria diastropha resembles Cercaria inhabilis closely the 

 description will be limited to pointing out the differences. As in Cer- 

 caria inhabilis different stages of development of the cercaria are found 

 free in the snail. The mature cercariae extended and contracted their 

 bodies very actively but none were noted swimming freely. This may 

 have been due to the condition of the material studied. There was no 

 cheek on the one examination. 



Cercaria diastropha (Fig. 23) is cylindrical in cross section, pointed 

 anteriorly, and when not contracted or flattened the region in front 

 of the acetabulum is but little wider than that sucker. In living speci- 

 mens the body varied from 0.27 to 0.54 mm. in length according to the 

 state of contraction, and the width changed from 0.20 to 0.08 mm. The 

 tail is always shorter than the body, being 0.22 mm. to 0.38 mm. in 

 length and with an average width near its base of 0.054 mm., The tail 

 is attached dorsad to the acetabulum, which is terminal and forms a 

 flattened base for the conical body (Fig. 24). The oral sucker is elong- 

 ate and in a specimen of average contraction has a length as great or 

 greater than the acetabulum. In an animal about the state of con- 

 traction of Figure 23 the oral sucker had a length of 0.11 mm. and a 

 width of 0.065 mm. and the acetabulum had a diameter of 0.105 mm. 



The eye-spots are like those already described but are larger in 

 proportion to the size of the body than in Cercaria inhabilis. Except 

 for a very limited area around the eyes Cercaria diastropa is entirely 

 unpigmented. 



The body is filled with cystogenous glands from the oral sucker to 

 the acetabulum. 



On account of the freedom from pigmentation it was possible to 

 work out the excretory system further in Cercaria diastropha than in 

 Cercaria inhabilis. Figure 23, ex, shows the relations of this system. 

 The tail vessel and the bladder are alike in both forms. The much 

 convoluted crura of Cercaria diastropha which are large and contain 

 scattered concretions, extend as unbranched vessels up to the region 

 of the eyes. There they receive small branches from all parts of the 

 body. It was possible to trace the largest of these branches altho on 

 account of the cystogenous glands, the flame cells and the smallest 

 ducts were not found. Into the tips of the crura open on each side 

 two vessels, one from the side of the oral sucker and one from the 

 posterior end. The posterior branch is soon divided into an outer and 

 an inner vessel, which subdivide to reach all parts of the posterior body 

 region. 



