60 ILLINOIS BIOLOGICAL MONOGRAPHS [506 



In none of the individuals studied were the suckers fully developed 

 or functional. The oral sucker shows a beginning of a mouth cavity 

 but the acetabulum is merely a rounded off mass of embryonic nuclei. 

 In mounted specimens the oral sucker averages 0.027 mm. in diameter 

 and the acetabulum, which is back of the middle of the body, has a 

 width of 0.024 mm. 



On the surface of living specimens of Cercaria leptacantha weie 

 scattered highly refractive round globules of different sizes, which 

 appeared like water or oil droplets. These bodies, which resemble the 

 concretions in the excretory systems of certain cercariae, were so prom- 

 inent that they could be seen thru the walls of the sporocysts, but dis- 

 appeared in the preservation of the material. 



The small characteristic stylet (Fig. 79) is 0.011 mm. to 0.13 mm. 

 in length, and 0.0025 mm. in thickness at its base. 



Two kinds of glands were present in Cercaria leptacantha. The 

 first type consists of small irregular shaped bodies with granular con- 

 tents at the anterior lateral margins of the acetabulum. The other 

 type are almost clear globlet shaped stylet glands, four on each side 

 arranged along the body lateral to the acetabulum, with ducts from 

 their outer margins leading up to the cephalic spine. No ducts were 

 found for the first type of gland, but from their granular contents they 

 may be cystogenous in character. 



The digestive system of Cercaria leptacantha is represented only 

 by a short prepharynx and a small pharynx 0.09 mm. in diameter. 



Of the excretory system only the elongate, club-shaped bladder can 

 be distinguished. 



The anlage of the reproductive organs is represented merely by a 

 large mass of small nuclei dorsal and posterior to the acetabulum. 



Cercaria leptacantha belongs to a group of very small cercariae 

 which Liihe (1909:196) calls Cercariae Microcotylae. It is possible that 

 they form a natural group. They are, however, so insufficiently known 

 that no final judgment can be passed on their relationships. At present 

 it seems best to follow Liihe in considering them a provisional group, 

 with Cercaria microcotyla Filippi as the type and the following char- 

 acteristics. 



1. Developed in gastropods in round or oval sporocysts which are 

 seldom more than twice as long as wide. 



2. Cercariae under 0.2 mm. in length. 



3. Acetabulum back of the middle of the body and smaller than 

 the oral sucker. 



