2O2 THE ANIMAL PARASITES OF MAN 



In some species it carries long cilia, by aid of which the young 

 swim about when released from the egg-shell ; as a genera 

 rule, however, there are no cilia and the integument is homo- 

 geneous, or is composed of numerous rods and is calcified (fig. 130). 

 The exterior envelope (investing integument) lies close to the egg- 

 shell and remains within it when the ciliated embryo hatches 

 out, and in many species it perishes at the end of the embryonal 

 development with the delicate egg-shells, so that one observes not 

 the entire ovum, but only the embryo in its embryonal shell 

 (fig. 131, a). 



The embryo enclosed within the embryonal shell, the ONCOSPHERE, 

 is of spheroid or ovoid form (fig. 131, b), and is distinguished by 

 the possession of three pairs of booklets, a few terminal cells, and 

 is provided with muscles to move the hooklets. 



No FURTHER DEVELOPMENT of the oncosphere takes place either 



FIG. 131. a., Oncosphere, with shell 

 of Tcsnia africana greatly magnified) after 

 von Linstow) ; b., freed oncosphere of 

 Dipylidium caninum (greatly magnified) 

 (after Grassi and Rovelli). 



in the parent organism or in the open ; in fact, in all cases in 

 which the oncospheres are already formed within the proglottides 

 they do not become free, but remain in their enveloping sheath ; 

 it is only when the oncospheres are provided with a ciliated 

 integument, that they leave the egg-shell, and even cast off the 

 ciliated integument, after having swum about in water for some 

 time. Sooner or later, however, all the oncospheres leave the 

 host that harbours the parental tapeworm and reach the open, 

 either still enclosed in the uterus of the evacuated proglottides, after 

 the disintegration of which they become free, or (as eggs with 

 shells) after being deposited in the intestine of the host, which 

 they leave with the faeces ; in the former case, the slightest injury 

 to the mature proglottides while still in the intestine suffices to 

 allow a part of the oncospheres with their integument to be 

 released and mingled with the faeces ; they have then been erro- 

 neously termed " eggs of Taeniae." 



In any case, however, the oncospheres must be transmitted into 



