60 ILLINOIS BIOLOGICAL MONOGRAPHS [60 



replace it in many forms of this genus, was inconspicuously developed, if present. 

 Personally, I incline to the view that on more careful examination this structure 

 will be found in all species, even those in which its absence has been made a 

 matter of record. Accordingly, not much weight can be put in its presence or 

 absence in any individual. 



"Three plerocercoid larvae or young cestodes were found in company with 

 Proteocephalus pusillus, which I regard as young forms of this species. The 

 largest came from the salmon which was most heavily infected with this cestode 

 parasite. It was 3.15 mm. long and had begun to assume clearly the appearance 

 of an immature cestode. The head measured 0.3 mm. wide by 0.26 mm. long, and 

 the suckers 0.14 mm. in length by 0.11 mm. in width. The neck was slightly nar- 

 rower than the head, but was not clearly set off from the body, which was very 

 uniform in diameter and measured 0.25 mm. in average width. The posterior end 

 of the body was swollen into a rounded knob about 0.35 mm. broad and of ap- 

 proximately the same length. This feature was evidently produced by a powerful 

 contraction of the terminal region of the body. In and near it one could see very 

 indistinct indications of proglottid formation. In form, size, and general aspect 

 this young cestode was in full agreement with the anterior regions of the mature 

 cestodes of this species with which it was associated. The head, which was not 

 contracted, showed on careful study the delicate outline of a rudimentary end 

 organ. While such a structure was not demonstrated in the mature individuals 

 described above, one can say positively that if present it could not have been seen 

 owing to the greatly contracted condition of the adult scolices. I believe that its 

 presence will be demonstrated in more favorable specimens. The complete agree- 

 ment of this largest larva with the mature specimens in all other features compels 

 me to regard both as different stages in the development of the same species. 



"The other larvae were still in early stages of development and probably had 

 been ingested by the salmon at a very recent date. Their relationship is not so 

 clear in all respects, and yet I do not hesitate to associate with the new species of 

 Proteocephalus a plerocercoid or young cestode obtained from the same host as 

 the adult worms and the older larva just described. The head is broadly conical, 

 without furrows, and measures 0.3 mm. in breadth. The suckers measure 60 to 

 74 fi in diameter. There is no rostellum or fifth sucker to be found, while the end 

 organ is so poorly developed as to be visible with difficulty and only under the 

 most favorable circumstances. The neck is nearly as broad as the head. In gen- 

 eral appearance this larva resembles the adult cestode and the older larva pre- 

 viously described. With some reserve one may also assign to this species a single 

 plerocercus taken from another specimen of Salmo sebago. The head, which meas- 

 ures only 150 fi in breadth, is shaped like that of the young cestode and like it is 

 without rostellum or fifth sucker, while the end organ is difficult to demonstrate. 

 Neither furrows nor ridges are seen on the larva, which has a total length of 1.14 

 mm. The sucker measures only 30 to 45 fx in diameter. The neck is slightly nar- 

 rower than the head. This form certainly belongs to the genus Proteocephalus 

 and probably to the species already described." 



