82 ILLINOIS BIOLOGICAL MONOGRAPHS [82 



Moreover it scarcely seems probable that Perca fluviatilis and Corego- 

 71US fcra normally harbor the same parasitic species. With this doubt 

 in mind some material which Dr. H. B. Ward has received from Pro- 

 fessor Parona labelled Taenia ocellata from Perca fluviatilis was care- 

 fully investigated. It was found that this material which at first 

 glimpse appeared to be identical with P. fallax La Rue from Coregonus 

 fera was after all sufficiently different to make it a separate species 

 which was described by La Rue (1911:476) from material on which the 

 present description is based. Material studied was alcoholic material 

 in Professor Parona 's collection labelled ^'Taenia ocellata, Perca fluvia- 

 tilis," locality not known, and slides and alcoholics of the same in Prof. 

 H. B. Ward's collections. 



These cestodes are of small size, being short and slender. The 

 longest specimen in Professor Parona 's material measured about 40 

 mm. in length by about 0.80-1.2 mm. in maximum breadth. In such a 

 specimen there are about 30 proglottids with eggs in the uterus, 12-18 

 mature proglottids, about 30 which show more or less developed sexual 

 organs, and very few in which the anlagen of the sexual organs cannot 

 be seen, in all about 80-90 proglottids. Segmentation is not very dis- 

 tinct for the proglottids are joined by nearly their entire breadth. The 

 corners are slightly rounded. Zschokke stated that the margins of the 

 proglottids are not rounded and their angles are somewhat rounded; 

 that the animal never presents a crenated appearance and that it has 

 the appearance of a ribbon. Zschokke found from 60-100 segments in 

 the strobila. His longest specimen was about 4 cm. long and about 1 

 mm. broad. 



The head (Figs. 147, 148) is of small variable shape, well set off 

 from the neck. Frequently its anterior face is somewhat conical, at 

 other times flattened. The head is flattened dorsoventrally. Its length 

 varies from 0.100-0.140 mm. Its breadth is from 0.127 mm. in heads 

 turned slightly sidewise to 0.212 mm. in slightly swollen heads. At its 

 broadest part the head bears four suckers which are directed outward 

 and upward. These suckers appear to be variously placed on the head 

 depending on the state of contraction. A small fifth sucker 0.026-0.037 

 mm. in diameter is situated at the apex of the head. The four suckers 

 vary in size from 0.069-0.080 mm. in diameter. They are usually nearly 

 circular in outline but may be a little irregular. The cavity is deep 

 and irregular in shape or it may be nearly round in outline. The neck 

 is slender, 1.8 mm. to 3.0-3.5 mm. in length by 0.085-0.140 mm. in 

 minimum breadth. The length of the neck was determined by measur- 

 ing from the head to the first segmentation visible in stained prepara- 

 tions. 



