THE CESTO1DEA 



135 



same margin throughout the strobila, when they are " unilateral " 

 (on the left side in Hymenolepis), or in some proglottids they are on 

 the right margin, in others on the left margin, when they are said 

 to be " alternate " ; but it is only rarely that there is any approach 

 to a regular alternation. In opposition to what obtains in the 

 Tetraphyllidea, the penis generally lies anteriorly to the vagina, 

 except in Ichthyotaenia and Mesocestoides. 



The female generative organs themselves present certain differ- 

 ences in structure and arrangement, as will be gathered from a 

 comparison of the typical proglottids figured ; but the most im- 

 portant difference is presented by the vitellarium, which, instead of 

 being an extensive follicular organ, is except in Mesocestoides and 

 Ichthyotaenia, and some species of Taenia a small acinous gland 

 situated behind the germarium. The uterus, as in all the Tetraboth- 

 ridiata, is a median sac, formed as an outgrowth of the germ duct ; 

 it is here more or less deeply notched, or even prolonged laterally 

 into branching " egg sacs," the shape of the whole organ forming a 

 specific character. In these forms, like Moniezia, with very short 

 but wide proglottids, the uterus becomes transverse (Fig. XXV.). 



In some cases the wall of the 

 uterus disappears as the eggs 

 ripen, and these, either singly or 

 in groups, come to lie freely in 

 the parenchyma, the cells of 

 which form capsules around each 

 group (T. dispar, Davainea, etc.). 

 In a few instances it is stated 

 that the germarium actually be- 

 comes the uterus (D. struthionis). 



A very interesting pheno- 

 menon is the duplication of the 

 genital organs (as in some species 

 of Bothriocephalus) ; thus in 

 Moniezia there are two complete 

 sets of organs in each proglottid 

 (Fig. XXV. 1); in Dipylidium 

 the uterus is single, but the 

 other organs are duplicated 

 (Fig. XXVI.) ; whilst in Ama- 

 bilia there is a single set of FIG. xxvi. 



female Organs, but tWO penes in A mature proglottid of Dipylidium caninum, 



Y ,,. -, L., out of the cat. (Orig. x35.) a, the uterus, 



proglottid. here broken up on to a number of independent 



TV/,* 7^'o^/v. TV. 4- sacs : b, one of the pair of genital pores; c, the 



Life-history. Ihe Segmenta- cirT ^ s -' d> vagina; e, the ventral excretory 



Of the egg and the develop- can ^ J / the transverse canal ; g, the lateral 



ment of the hexacanth embryo 



has been traced out by Leuckart, E. van Beneden, and Moniez, 



