Fig. 41.—Diagrammatic transverse section of Schistocephalus solidus Crepl., from the Wild-duck, illustrative of the Cestodes with uterine aperture (uto). × 12. cs, Cirrus-sac; for.sec, one of the paired lateral openings of the excretory vessels; ln, longitudinal nerve; ov, ovary; ovd, oviduct; par.m, parenchymatous muscles; r.sem, receptaculum seminis; sh.gl, shell-gland; te, testes; ut, uterus; uto, uterine pore; vag, vagina; vd, vasa deferentia; yd, yolk-duct; yg, yolk-glands (black); ♂, male, ♀, female genital aperture. (After Riehm.)
The reproductive organs, unlike the preceding systems, are discontinuous from one proglottis to the next. The male and female organs and their mutual connexions, especially in the unsegmented Cestodes, may be compared in detail with those of Trematodes, but the difference between the arrangement of the generative organs of various Cestodes is very great.[[107]] The penis (Fig. 41, cs) is evaginated through the male pore (Fig. 41, ♂), and inserted far into the vagina (♀, vag) of the same or another segment of the tape-worm.
Fig. 42.—A, Free-swimming, six-hooked larva of Bothriocephalus latus Brems. (the broad tape-worm of Man), still enclosed in a ciliated (possibly cellular) double membrane or mantle. In this condition it may continue to live in water for a week or more, but eventually throws off its ciliated coat (as in B) and commences to creep about vigorously by the aid of its hooks, in search of its first host, which is at present unknown. (After Schauinsland.) × 600.
From this fact and the anatomical relations of the vagina, it is becoming increasingly probable that the so-called uterus of Trematodes is an organ corresponding to the vagina of Cestodes, and not to the uterus of Cestodes. The latter opens to the exterior in Schistocephalus, Bothriocephalus, and some other Cestodes of fishes by a special pore (Fig. 41, uto). Through this, some of the eggs (which in these genera give rise to ciliated larvae) are enabled to escape, and need not wait for the detachment of the proglottis, as must happen in the Taeniidae, where the uterus is closed. This uterus, a true physiological one, is probably the homologue of the "canal of Laurer" ("Laurer-Stieda canal," or "vagina") of Trematoda. The fertilised ovum and yolk are brought together into the "ootype," where the shell-gland forms the egg-shell around them (Fig. 41, sh.gl) and the egg is then passed into the uterus. The ovum segments to form a minute six-hooked larva, which may (Bothriidae, Fig. 42) or may not (Taeniidae) be ciliated. Thus in Taenia serrata the proglottides are shed with the faeces of the host (dog), and they protect the young from the desiccating influence of the surroundings. If inadvertently eaten by a rabbit along with herbs, the proglottis and larval envelope are digested, and by its six hooks the tiny larva bores through the gastric wall into the portal vein, and so into the liver. Here the hooks are thrown off, and the solid mass of cells becomes vacuolated.
Fig. 43.—Stages in the development of Dipylidium caninum L. (= Taenia elliptica Batsch, T. cucumerina Bloch), the commonest of the Dog-Taeniae; compare Fig. 44. A, Six-hooked larva (now often spoken of as an "Onchosphaera"); B, larva elongating; formation of a central lacuna; C, larva further advanced; D, distinction between body and tail is visible; E, invagination of the rostellum is commencing; F, Cysticercoid larva with four suckers, invaginated rostellum, and excretory vessels. c, Calcareous concretions in cells of the larva; ex.o, excretory aperture; ex.v, excretory vessels; inv, invagination commencing; rost, rostellum; sc, suckers. (After Grassi and Rovelli; highly magnified.)
Fig. 44.—Schematic longitudinal sections through the larvae of Dipylidium caninum L. All these stages are passed in the body-cavity of the Dog-flea (Pulex serraticeps). (Compare Fig. 43 for further details.) A, Six-hooked larva with developing rostellum (shaded) and suckers (black). In this species the invagination (C, invag.) occurs after the formation of these organs, and not, as in most Taeniae, before it. B, Invagination commencing; the hooks are developing above the rostellum, while beneath it the nervous system (dotted) is seen. C, The invagination has now carried the suckers inwards. The tail has become distinct, and the whole larva at this stage is known as a Cysticercoid. hk, Larval hooks; invag, mouth of the invagination; n, central nervous system; rost, rostellum and hooks; sc, suckers, of which only two can be seen in a longitudinal section; four are really present. (After Grassi and Rovelli.)
At one pole an invagination occurs, at the bottom of which the rostellum, suckers, and hooks are gradually formed, but inside out as compared with the head of the Taenia serrata. At this stage the larva (Cysticercus pisiformis) has usually issued from the liver and attached itself to the omentum. The invagination projects into the cavity of the bladder, within which a watery fluid accumulates. Thus the "bladder worm" is formed, the head of which is evaginated if the larva be introduced into the digestive system of a dog. The bladder and neck of invagination are digested, while the head, protected by these, remains, and forms the neck, from which the proglottides are afterwards segmented off. In Taenia (Hymenolepis) murina the whole development may take place in the parental host, the larva living in the villi, the adults in the cavity of the same rat's intestine (Grassi). The different forms of Cestode larvae depend largely upon the presence and degree of development of the caudal vesicle or bladder, which in Scolex polymorphus (Fig. 38) (the young stage of Calliobothrium filicolle Zsch.) is practically absent. If the bladder be small, the larva is known as a Cysticercoid. For example, the common Dipylidium caninum, which lives in the dog, has such a larva, the development of which is explained and illustrated by Figs. 43 and 44. The bladder becomes exceeding capacious in Coenurus and Echinococcus.