Genital Organs.—With the exception of one genus (Diœcocestus, Fuhrm.), in which the species are sexually differentiated, all the Cestodes are hermaphroditic; the genitalia develop gradually in the segments (never in the scolex), the male organs, as is usual in hermaphroditic animals, forming earlier than the female. The youngest proglottids generally do not exhibit even traces of genitalia: these, as a rule, develop first in the older segments, and the development proceeds onwards from segment to segment. In a few exceptional cases (Ligula) the sexual organs are already developed in the larval stage, but are only functional after the entry of the parasite into the final host.
Fig. 191.—Proglottis of Tænia saginata, Goeze, showing genitalia. C., transverse excretory canal; N., lateral longitudinal nerve; W., longitudinal excretory canal; T., testicles scattered throughout the proglottis; Ut., opposite the central uterine stem (a closed sac); Ss., genital pore leading into the genital sinus; above the cirrus and coiled vas deferens (V.d.), below the vagina (Vag.), bearing near its termination a dilatation, the seminal receptacle; Vsc., the triangular vitellarium, and above it (Shg.) the shell gland; leading from this to the uterus is seen the short uterine canal, on either side of this the two lobes of the ovary (Ov.). 10/1.
With the exception of the end portions of the vagina, cirrus and uterus, all the parts of the genital apparatus lie in the medullary layer, except only the vitellaria, which in many species are in the cortical layer. The male apparatus consists of the testes, of which, as a rule, there are a large number,[278] and which lie dorsal to the median plane (fig. [185], T.); a vas efferens arises from each testis, unites with contiguous vasa, and finally discharges into the muscular vas deferens that is situated in about the middle of the segment. According to the position of the genital pore, the vas deferens opens on the lateral margin or in the middle line in the front of the segment; it is much convoluted or twisted, and frequently possesses a dilatation termed the vesicula seminalis. It finally enters the cirrus pouch, which is usually elongated; within the cirrus pouch lies the protrusible cirrus, which is not uncommonly provided with hooklets.
Fig. 192.—Dibothriocephalus latus. Upper figure: female genitalia, ventral view. Lower figure: male genitalia, dorsal view. The central portion only of the proglottis is shown. a, cirrus sac; b, partly everted cirrus; c, genital atrium and pore; d, vaginal pore; e, uterus; f, uterine pore; g, vagina; h, ovary; i, shell gland; j, vitelline duct; k, lateral nerve; l, vitellarium; n, vas deferens (muscular portion); p, vas deferens; q, seminal vesicle; r and x, vasa efferentia; s, lateral excretory canal; t, testicular follicles. (After Benham and Sommer and Landois.)
The male sexual orifice almost always opens with that of the vagina into a genital atrium, the raised border of which rises above the edge of the segment and forms the genital papilla (fig. 191).