Page:EB1911 - Volume 26.djvu/440

Rh each with a scolex, may arise from a single embryo. We have, in fact, a form of larval multiplication that recalls the development of digenetic Trematodes.

The eggs of Cestodes consist of oval or spherical shells ($1⁄500$ in. diameter), containing a fertilized ovum surrounded usually by many yolk-cells. The shell is thick, and operculate in some forms; thin, and provided with filaments, in others; in the latter cases it may contain only a few yolk-granules suspended in an albumen-like substance. The development of the six-hooked embryo or “onchosphere” takes place in the uterus. The ovum first divides into (a) a granular cell, and (b) a cell full of refringent spherules. The former divides into (c) small cells or micro meres, and (d) large cells or megameres. (c) forms the body of the embryo, (b) and (d) enclose it and form a covering. The embryo undergoes differentiation into an outer layer of cells that produce a chitinoid coat,

a middle layer of cells, and a central spherical hexacanth body closely enveloped by the middle coat. In a few genera the place of the chitinoid coat is taken by a ciliary investment and in most families the structure of the layers is characteristic.

Arrived in the intestine of the intermediate host, the hooked embryo is set free and works its way to some distant site. Here it undergoes a change into a cystic or “metacestode” state. A cavity appears in its centre and it acquires a pyriform shape. The thicker portion develops a terminal muscular rostellum and two or four suckers, the thinner end (“tail”) is vesicular, more or less elongated, and contains the six embryonic hooks. By a process of infolding, the thicker end is partially invaginated, the middle portion or “hind-body” and the organism may now present a superficial likeness to a cercaria. An excretory system develops, opening at the base of the tail; nervous and muscular systems arise; and finally the rostellum and suckers become completely enclosed in the sac formed by the lateral extension of the “hind-body.” When swallowed by the final host such a “cysticercoid” larva evaginates its scolex, throws off its hooked vesicular tail, and begins to bud off proglottides at its free end (fig. 10).

Such is the general history of Cestodes whose intermediate host is an Invertebrate. In most other cases the tail is not distinguishable, and the body of the larva is separable only into a scolex invaginated with a bladder ( = hind-body and tail). This form of larva is known as a cysticercus. In some genera a “urocyst” is formed, the tail of which gives rise to a new cyst and a fresh scolex.

The most remarkable feature of this cystic development is the formation in many genera of several internal buds within a common cyst, each of which forms an independent inverted scolex (Coenurus,