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 (Cœnurus, 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.
The above development is to be regarded as a case of complicated metamorphosis secondarily produced by the necessities of a parasitic condition, to which an alternation of sexual and gemmiparous generations has been added. The alternation of generations only occurs at the last stage of the development, when the so-called head, without generative organs, produces by budding a chain of sexual forms, the embryos of which, after passing through a complicated metamorphosis, again become Cestode heads.