134 



THE CESTOIDEA 



bearing the booklets at its apex ; this rostellum is retractile into 

 a second muscular sac, or " receptaculum rostelli." In other cases 

 the rostellum carries a sucker. In any case the rostellum is retractile, 

 and the arrangement of its musculature presents certain differences 

 which appear to be characteristic of each sub-family (Liihe, 26). 



In a few peculiar instances the scolex has been described as 

 being absent, as in T. malleus and Idiogenes otidis (both rare forms), 

 where the anterior proglottids become modified to form a " pseudo- 

 scolex" (cf. Thysanoceplialum). The size of the strobila, and the 

 number of proglottids, exhibit that same range of variation referred 

 to in the other groups ; for instance, in T. echinococcus there are but 



FIG. XXV. 



1. A proglottid of Moniezia expansa, Rud. (after Stiles and Hassall). 2. Mature proglottid 

 of Hymenolepis diminuta, Rud. (after Zschokke), out of the rat. 3. Ditto of Thysanosoma, 

 ovilla, Riv., out of sheep, a, uterus ; b, genital pore ; c, cirrus ; d, vagina e, germarium ; 

 /, vitellarium ; h, testis ; g, proglottidean glands ; x, excretory canal. 



four proglottids; in T. saginata 1200 or more. The shape of the 

 proglottids, both before and after separation, is also a valuable 

 specific character ; but here the development of the free proglottids, 

 which, as a rule, drop off in groups, 1 is but slight, since the eggs in 

 the uterus have already developed into hexacanth embryos before 

 the separation. The shape too of the uterus, filled with ripe eggs, 

 is employed as a specific character, while the egg coverings vary in 

 each of the sub-families of Taeniidae. 



The genital pore is marginal, but Mesocestoides forms an excep- 

 tion, the male and female pores being separate, and on the ventral 

 surface. The genital pores, as in Tetraphyllidea, are either in the 



1 In Anoplocephala and Drepanidotaenia spp. no proglottids drop off. The 

 strobila retains its entirety throughout life. 



