180 



EMBRYOLOGY 



intestine, have separated from those lying between them, the 

 latter can be considered as belonging to a third germ-layer, 

 the mesoderm. 



When the embryo has reached the stage described, it 

 breaks through the enveloping membrane, which has become 

 a delicate transparent pellicle, the operculum of the egg-shell 

 opens, and the embryo reaches the outside world (Fig. 87 F 

 and G). Here it creeps abdut actively, and for this purpose 

 it makes especial use of the proboscis. The anterior part of 

 the intestine has become metamorphosed into an organ of 

 this kind, for it can be everted and retracted. In the embryo 

 represented in Fig. 87 H, the proboscis, together with the 

 anterior end of the body, is retracted. A kind of funnel 



arises in this way which is 

 surrounded by the chitinous 

 bristles. 



The embryos of other DutoviidcB 

 develop on the cells of the ectoderm 

 cilia, by means of which they swim 

 in the water (Figs. 88 and 89 A). The 

 development of an enveloping mem- 

 brane was observed by Schauinsland 

 in various genera of Butomida. Two 

 Bistomidce (D. cylindraceum and D. 

 mentulaiiim) on leaving the egg-shell 

 appear to cast off the ciliated ecto- 

 dermal layer in addition to the en- 

 veloping membrane. 



We shall meet with similar pro- 

 cesses in considering the development 

 of tapeworms. The significance of 

 this will be considered there more at 

 length. 



Fig. 88.— Embryo of Biytomum 

 glohiporum, pressed out of the egg 

 (after Schauihslaxd). The ectoder- 

 mal cells (Ec) are partly detached ; 

 Km, enveloping membrane. 



The Further Course of 

 Development. — The distomid 

 larva, in order to develop further, must seek another host. 

 The processes which are enacted during its growth we de- 

 scribe first for Distomum hepaticum , whose course of develop- 

 ment has been made known chiefly through the prolonged 

 investigations of Leuckart, as well as those of Thomas. 



