191] PROTEOCEPHALIDAE—LA RUE 191 



broad. "The elongated shell is very thin and surrounds in part the 

 yet undifferentiated egg-cell with the yolk-cells, in part the cell-groups 

 which are the results of the first development stages." Evidently 

 Riggenbach saw no embryos. 



In life the heads of typical species of Proteocephalus are extremely 

 variable in form passing with considerable rapidity from one contraction 

 state to another. By the contraction of longitudinal muscles extending 

 into the apex of the head the suckers may be drawn down out of sight 

 within the inflated neck region but there are no folds of tissue at the 

 base of the head within which the head may retreat. In the species 

 just described this fold of tissue within which the suckers are partially 

 withdrawn seems to be a constant feature of the scolex. Thus the head 

 differs from heads of species of Proteocephalus. At first one notes cer- 

 tain similarities with the scolices of species of Corallobothrium. Fur- 

 ther consideration of the structure of the two types of heads convinces 

 one that they are not alike. The head of a Corallobothrium is greatly 

 flattened anteriorly and the suckers are directed anteriad. At the mar- 

 gins of the flattened apical region are numerous folds and lappets which 

 form a corolla-like sheath about the suckers. In this species the head 

 is conical and the suckers are directed outward. In Corallobothrium 

 lobosum Riggenbach (1896) found a small muscle-cross connecting each 

 dorsal sucker with the ventral sucker opposite it. Riggenbach did not 

 see such a structure in his sections of the heads of Ichthyotaenia ab- 

 scisa. It is doubtful if it occurs in any other genus known at present 

 to belong in this family. 



This species may be considered as forming a transitional stage 

 between Proteocephalus and Corallobothrium. This view is supported 

 by the appearance of heads of plerocercoids of an unknown species of 

 Corallobothrium found encysted in the liver of Ameiurus melas and A. 

 nebulosus from the Illinois river. The heads of these plerocercoids pos- 

 sessed but a few simple folds of tissue enveloping a part of the head. 

 When alive the heads were somewhat conical, not flattened on the apex 

 as in preserved heads of the adult worms. However, a marked difference 

 between these heads and the heads of Choanoscolex abscisa may be noted. 

 In the plerocercoids the suckers are plainly paired, two dorsal and two 

 ventral, while in this species the head is but slightly flattened dorso- 

 ventrally and the suckers are not plainly paired. This species does not 

 belong in the genus Corallobothrium or Proteocephalus. 



The foregoing descriptions of species of Proteocephalus are briefly 

 summarized in and supplemented by the following comparative table. 



