158 ILLINOIS BIOLOGICAL MONOGRAPHS [158 



protuberance is greatly contracted. Between the suckers are deep 

 grooves which in many cases cause the head to appear almost lobate. 

 The suckers themselves are more or less variable in shape but they 

 always show the pointed apex. The cavity is shallow, the muscular 

 wall thin. The suckers measure 0.130-0.170 mm. long by 0.170-0.190 

 mm. broad. A fifth sucker is not present. A careful search through 

 three series of sections has failed to reveal any trace of an endorgan. 

 The neck is long and slender, 2.0-3.0 mm. to the first traces of segmenta- 

 tion. The breadth is 0.1-0.2 mm., usually much narrower than the head. 

 The neck grades over almost imperceptibly into the first proglottids 

 which are very difficult to discern. 



The first proglottids are 12-15 times broader than long, being about 

 0.255 mm. broad by 0.017-0.020 mm. long. As the proglottids near 

 maturity they increase greatly in size measuring up to 0.85 mm. broad 

 by 0.34-0.37 mm. long. Proglottids in which the uterus contains some 

 eggs measure 0.51 mm. long by 0.867 mm. broad or are even quadrate, 

 each dimension being about 0.85 mm. The ripest proglottids become 

 longer than broad, measuring 0.680 mm. broad by 0.90-1.00 mm. long 

 and in rare cases of old spent proglottids the length may be about 2.0 

 mm. and the breadth about 0.4 mm. Segmentation is evident. No lon- 

 gitudinal furrows are present. Transverse furrows occur only at the 

 division lines between the segments. The posterior angles of the pro- 

 glottids project slightly beyond the anterior margin of the proglottid 

 following. 



This species has not been thoroly investigated as to its histological 

 structure. However, certain points have been worked out with some 

 care. The cuticula, the subcuticula, the parenchyma, the muscles of the 

 strobila, and the nervous system were not seen to differ essentially from 

 similar structures in P. ambloplitis. 



The muscles of the head were worked out in series of transverse 

 and frontal sections. The muscles of the apical protuberance of the 

 head could not be worked out with care but that region could be seen 

 to be well supplied with muscles especially in the subcuticular layer. 

 The peripheral parts of the apical region of the head contain many 

 heavy muscles going up toward the tip. In the middle and lower region 

 of the suckers about 0.135 mm. from the tip of the head heavy muscles 

 cross from one sucker wall to the wall of the sucker opposite forming 

 a heavy muscle cross (Fig. 40). "Weaker muscles cross the head dorso- 

 ventrally and laterally. These muscles extend to the subcuticular re- 

 gion and serve by their contraction to cause in part the deep grooves 

 between the suckers. Near the upper limits of the suckers at a depth 

 of about 0.10 mm. from the tip of the head heavy muscles (Fig. 39) 



