PLATHELMINTHES 197 



volume. The Cysticercus of Echinococcus, which remains foi- 

 several months in this stage, attains during this period about 

 the size of a walnut, but, as is well known, it may become 

 much larger ; that of Tcenia coinurus grows in five weeks to 

 the size of a pea. Most of the Cysticerci reach in the course 

 of three weeks or so the diameter of about 1 mm. Then a 

 rapid cell-growth is noticeable at the anterior pole. This 

 gi'ows inward in the form of a knob (Fig. 96 B and G). 

 Corresponding to the cell-proliferation, there is a pit-like 

 depression on the surface of the vesicle, Avhich increases in 

 depth with the growth of the knob. The entire gi^owth 

 represents the fundament of the head of the tapeworm 

 (scolex), which therefore arises as an invagination of the wall 

 of the vesicle (Fig. 96 Bto F). It appears that the want of 

 space, to which the tapeworm is subjected as the result of 

 its mode of life, has the effect of preventing the scolex from 

 arising as an [external] appendage of the body, as would 

 seem most natural, and causes it to be formed as an invagina- 

 tion of the vesicle, which is only subsequently evaginated. 



The suckers arise as pit-like depressions of the lateral 

 walls of the cavity of the cephalic knob, and the hooks of 

 the head of the tapeworm are developed at the bottom of 

 this cavity (Fig. 96 D). The head is now completely formed 

 in negative. Beginning with the deepest part, the future 

 rostellum (Fig, 96 U), the head is completely reversed by 

 evagination (Fig. 96 F), and thus attains its permanent 

 form. It then appears as an evagination of the vesicle, 

 which is attached to its posterior end (Fig. 96 F). 



Before the later developmental processes and the meta- 

 morphosis into the tapeworm can be completed, it is neces- 

 sary for the Cysticercus to enter into another animal. This 

 takes place by its host being eaten in part or in whole by 

 the final host of the tapeworm. In the stomach of the final 

 host the scolex loses the caudal vesicle by its being digested. 

 In Fig. 96 Ö a small remnant of the bladder is still seen 

 attached to the scolex, which has just become free. The 

 scolex usually passes farther back in the intestine, sinks its 

 hooks and suckers into the mucous membrane, and upon the 

 appearance of segmentation becomes an adult tapeworm 



