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. Thysanocephalum). 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. Aproglottid of Mnniczia crjwnsa, Rud. (after Stiles and Hassall). 2. Mature proglottid 

 of Hymcndepis di-minnta, Rud. (after Zschokke), out of the rat. 3. Ditto of Thysanofvnni 

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

 /, vitellariuin ; A, 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 Mesocest&ides 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 AnctplocephaJa and Drepanidotaenia spp. no proglottids drop oft'. The 

 strobila retains its entirety throughout life. 



