92 ILLINOIS BIOLOGICAL MONOGRAPHS [92 



of vas deferens, by the difference in the size of ovaries, and by the 

 shape of the ovarial space in the ripe proglottid. From P. longicoUis, 

 as described by von Linstow, P. cernuae is distinguished by the much 

 shorter neck, the different position of the genital opening, by the num- 

 ber of testes, by the position and shape of the mass of coils of the vas 

 deferens, by the number of uterine pouches, and by the size of the eggs. 

 No American species yet discovered resembles it closely. 



This species is closely allied to the form which Zschokke designated 

 as P. longicollis (Rud.) but which has been determined to be a separate 

 species, P. neglectus. Unfortunately the heads of P. cernuae and P. 

 neglectus cannot be compared due to lack of heads in the latter mate- 

 rial. The shape and size of the proglottids may be about the same. In 

 P. neglectus the segmentation is evident but very indistinct in this spe- 

 cies. The minimum length of the cirrus-pouch of P. neglectus is greater 

 than the maximum of the same organ in P. cernuae. The ductus ejacu- 

 latorius in P. neglectus is straight but it has from two to three coils in 

 this species. In ripe proglottids of P. neglectus the ratio of the length 

 of the cirrus-pouch to proglottid breadth is 1 :3 to 2 :9 but in P. cernuae 

 it is 1 :5, 1 :6, 1 :7. 



The coils of the vas deferens are massed close to the cirrus pouch 

 in P. cernuae but they are mostly median in P. neglectus. The testes of 

 P. cernuae are nearly double the size of those in P. neglectus, so also 

 the length of the ovary in ripe proglottids is much greater in the former 

 than in the latter. The extent and compactness of the vitellaria and 

 their relation to the margin of the proglottid are different in the two 

 species. There may be more uterine outpocketings in P. cernuae than 

 in P. neglectus. The embryo measures 0.026-0.0265 mm. in P. neglectus 

 and about 0.021 mm. in spherical embryos of P. cernuae. A maximum 

 measurement of 0.0265 mm. may occur in the latter species but only in 

 elongated embryos. These species which seem much alike at first glance 

 are thus after all quite different. 



