PLATYELMINTHES. 213 



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. Ccenurus 

 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 con- 

 nective-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 liinacis, 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. 



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 maybe compared in almost 

 every respect with the sporocyst of a Trematode (Huxley). 



The next important change consists in the development of a 

 head, which becomes the head of the adult Tsenia. This is 

 formed in an involution of the outer wall of the anterior ex- 

 tremity of the cystic worm. This involution forms a papilliform 

 projection on the inner surface of the wall of the cystic worm, 

 with an axial cavity opening by a pore on the outer surface. 

 The layer of cells forming the papilla soon becomes divided 

 into two laminae, of which the outer forms a kind of investing 

 membrane for the papilla. The papilla itself now becomes 



