The central mass of cells becomes developed, as in the other type, into a six-hooked (rarely four-hooked) embryo ([fig. 96] G), but the superficial layer separates from the central, and either disappears or becomes (Bothriocephalus proboscideus) a cuticular layer. Between the six-hooked embryo and the outer layer of cells one or more thick membranes become deposited (E. van Beneden). The eggs are carried out of the alimentary canal in the proglottis and transported to various situations on land or in water. They usually remain within the proglottis, invested by their thick shell, till taken up into the alimentary canal of a suitable host, or they may be swallowed after the death and decay of the proglottis. They are subsequently hatched after their shell has become softened by the action of the digestive fluids.
Fig. 96. Diagrams of various stages in the development of the Cestoda.
(From Huxley.)
A. Cysticercus. B. and C. Cysticerci in the everted (B) and inverted (C) condition. D. Cœnurus. E. and F. Diagrams of Echinococcus. It is most probable that Tænia heads are not developed directly from the wall of the cyst as represented in the diagram. G. Six-hooked embryo.
Before proceeding to describe their further history, the close resemblance between the first developmental stages of Cestoda, especially in the case of the ciliated larvæ, and those of Trematoda, may be pointed out.
In both there is a ciliated larva, and in both there is developed within the ciliated skin a second larva, which becomes freed by the stripping off of the ciliated skin.
The type of development has moreover many analogies with that of the Nemertine larva of Desor, p. 163 (cf. Metschnikoff), and is probably like that an abbreviated record of a long history.
The suitable host for the six-hooked embryo to enter is rarely the same as the host for the sexual form. The embryos having become transported into the alimentary canal of such a host, and become free, if previously invested by the egg-shell, soon make their way, apparently by the help of their hooks, through the wall of the alimentary tract, and are transported in the blood or otherwise into some suitable place for them to undergo their next transformation. This place may be the liver, lungs, muscles, connective tissue, or even the brain (e.g. Cœnurus cerebralis in the brain of sheep).
Here they become enclosed in a granular deposit from the surrounding tissues, which becomes in its turn enclosed in a connective-tissue coat. Within lies the solid embryo, the hooks of which in many cases disappear or become impossible to make out. In other forms, e.g. Cysticercus limacis, they remain visible, and then mark the anterior pole of the worm ([fig. 98], c.). The central part of the body next becomes transformed into a material composed of clear non-nucleated vesicles. Accompanying these changes the embryo grows rapidly in size; a cuticle is deposited by its outer layer, in which also an external layer of circular muscular fibres and an internal layer of longitudinal fibres become differentiated, and internal to both there is formed a layer of granular cells.