62 ILLINOIS BIOLOGICAL MONOGRAPHS [508 



Cercaria brevicaeca (Fig. 74) was elongate oval in shape and its tail, 

 which was very easily lost, had about the same length as the body. 

 At average extension the body had a length of about 0.30 mm. and a 

 width of 0.14 mm. The tail did not change its shape very greatly and 

 ranged in length only from 0.22 mm. to 0.38 mm. with a width at its 

 base of 0.038 mm. It was attached in a groove in the ventral side of 

 the posterior end and when contracted had a tendency to curl at its 

 tip. 



The oral sucker of Cercaria brevicaeca had an average diameter 

 of 0.082 mm. and the acetabulum, which is just back of the middle of 

 the body, was slightly larger, 0.087 mm. 



The stylet (Fig. 73) had a length of 0.018 mm. and was slightly 

 thickened 0.007 mm. from its point. 



The body back to the acetabulum was covered thickly with rows 

 of very tiny spines. 



In the region between the acetabulum and the pharynx were two 

 clumps of from ten to twelve stylet glands, varying in length from 

 0.026 mm. to 0.035 mm. and in width from 0.018 mm. to 0.025 mm. 

 The ducts from these glands united into two groups one on each side 

 which passed dorsad of the oral sucker to open beside the stylet. 



Almost every bit of available space behind the pharynx was filled 

 with cystogenous glands, which from the surface appeared as round, 

 granular bodies 0.014 mm. to 0.017 mm. in diameter. 



The oral cavity was followed by a short prepharynx and a small 

 pharynx, 0.030 mm. in diameter. The short, narrow esophagus divided 

 just in front of the acetabulum into short intestinal ceca which did not 

 reach beyond the acetabulum. The lumina of the ceca showed only 

 as irregular, elongate spaces in the granular contents." 



Of the excretory system only the peculiarly shaped vesicle could 

 be made out. This was composed of a pyriform median portion, and 

 two more narrow lateral parts which almost completely surrounded the 

 acetabulum. The pore opened dorsad at the base of the tail. 



No cercariae were found in the literature closely corresponding with 

 Cercaria brevicaeca. Especially unique is the shape of the excretory 

 vesicle. 



THE CLASSIFICATION OF THE CERCARIAE 



At the present state of our knowledge it is impossible to fit most 

 of the cercariae into the general trematode classification, for, except 

 in those forms like the echinostomes or the amphistomes, where the larvae 

 are much like the adults, or in a group like the family Gorgoderinidae, 

 where the life histories of several species have been worked out, little 



