Cysticerci may remain a long time without further development, and human beings have been known to be infested with an Echinococcus cyst for over thirty years. When however the Cysticercus with its head is fully developed, it is in a condition to be carried into its final host. This takes place by the part of one animal infested with cysticerci becoming eaten by the host in question. In the alimentary canal of the final host the connective-tissue capsule is digested, and then the vesicular caudal appendage undergoes the same fate, while the head, with its suckers and hooks, attaches itself to the walls of the intestine. The head and rudimentary trunk, which have been up to this time hollow, now become solid by the deposition of an axial tissue; and the trunk very soon becomes divided into segments, known as proglottides (fig. 99 A). These segments are not formed in the same succession as those of Chætopods; the youngest of them is that nearest to the head, and the oldest that furthest removed from it. Each segment appears in fact to be a sexual individual, and is capable of becoming detached and leading for some time an independent existence. In some cases, _e.g._ _Cysticercus fasciolaris_, the segmentation of the trunk may take place while the larva is still in its intermediate host. [FIG. 99. ECHINOCOCCUS VETERINORUM. (From Huxley.) A. Tænia head or scolex. _a._ hooks. _b._ suckers. _c._ cilia in water vessel. _d._ refracting particles in body wall. B. single hooks. C. portion of cyst. _a._ cuticle. _b._ membranous wall of primary cyst. _c._ and _e._ scolex heads. _d._ secondary cyst.] The stages in the evolution of the Cestoda are shortly as follows: 1. Stage with embryonic epidermis either ciliated (Bothriocephalus, etc.) or still enclosed in the egg-shell. This stage corresponds to the ciliated larval stage of the Trematoda. 2. Six-hooked embryonic stage after the embryonic epidermis has been thrown off. During this stage the embryo is transported into the alimentary tract of its intermediate host, and boring its way into the tissues, becomes encapsuled. 3. It develops during the encapsuled state into a cystic worm, equivalent to the sporocyst of Trematoda. 4. The cystic worm while still encapsuled develops a head with suckers and hooks, becoming a Cysticercus. In some forms (_Coenurus_, _Echinococcus_) reproduction by budding takes place at this stage. The head and trunk are known as the scolex. 5. The Cysticercus is transported into the second and permanent host by the infested tissue being eaten. The bladder-like remains of the cystic worm are then digested, and by a process of successive budding a chain of sexual proglottides are formed from the head, which remains asexual. [FIG. 99 A. TETRARHYNCUS. (From Gegenbaur; after Van Beneden.) A. Asexual state. B. Sexual stage with ripe proglottides.]