With the rapid growth of the body a central cavity is formed, which becomes filled with fluid, and the embryo assumes the form of a vesicle. At the same time a system of excretory vessels, sometimes opening by a posterior pore, becomes visible in the wall of the vesicle.
The embryo has now reached a condition in which it is known as a cystic- or bladder-worm, and may be compared in almost every respect with the sporocyst of a Trematode (Huxley).
Fig. 97. Cysticercus cellulosæ. (From Gegenbaur, after von Siebold.)
a. Caudal vesicle. c. Anterior part of body. d. head.
Fig. 98. Cysticercus with small caudal vesicle.
A. Head involuted. B. Head everted.