CESTODA. 



331 



and the first proglottis with fully developed organs gives us an 

 expression for the number of stages through which each segment 

 has to pass. The tape-worms are oviparous; either the embryo 

 develops within the egg-shell in the body of the mother, or the 

 development takes place outside the proglottis, for example in 

 water (Bothriocephalus). 



The eggs of the Cestoda are round or oval in shape and of small 

 size. Their envelope is either simple or composed of numerous thin 

 membranes, or else forms a thick and strong capsule, which in Tccnia 

 is formed of densely packed rods united by a connecting substance, 

 and presents in consequence a granular appearance. In many cases 

 the development of the embryo coincides with that of the egg- 

 r-.he]l, so that the egg at the moment that it is laid contains a 



a 



FIG. 268. Stages in the development of Tcenia solium to the Ci/xticercus stage (partly after R. 

 Leuckart). a, Egg with embryo. I, Tree embryo, c, Rudiment of the head as a hollow 

 papilla on the wall of the vesicle, d, Bladder-worm with retracted head, e, The same 

 with protruded head, magnified about four tunes. 



complete embryo with six, or more rarely, four hooks. In BotJirio- 

 cephalus the development takes place outside the proglottis during 

 the long period that the egg passes in water, and the embryo 

 leaves the egg as a ciliated larva (fig. 267, c). The development 

 of the embryo into the tape-worm probably never takes place 

 directly in the same medium in the intestine of the original host. 

 As a rule there is a complicated metamorphosis, which is sometimes 

 (Echinococcus, Cwnurus) connected with alternation of generations ; 

 the successive stages live in different localities, and usually find the 

 conditions necessary to their development in different species of 

 animals, between which they migrate, partly actively and partly 

 passively. The eggs usually leave the intestine of the host with 

 the proglottis, and are deposited on dunghills, on plants, or in the 



