STRUCTURE OF THE FREE-SWIMMING EMBRYO. 



725 



FIG. 387. Free-swimming embryo of 

 BothriocepJialus latus with the protoplas- 

 mic threads between the body and the 

 ciliated mantle. (After Schauinsland.) 



the embryos to the presence of protoplasmic threads, which extend in 



regular order between the two lamellae, and thereby simulate cells. 



Just as the two lamellae of the mantle 



are connected with one another by 



threads, so, too, do threads connect 



the embryo with the mantle, but the 



latter are considerably fewer in 



number and less strongly developed 



(Schauinsland). They are best seen 



in embryos which have lived for 



some length of time in the water, 



and in which the space below the 



mantle has been enlarged by the 



absorption of water. In the in- 

 terior of the embryonic body, which 



has a diameter of 0'045 mm., and is 



thus considerably larger than that of 



the human bladder- worm, one recog- 

 nises in successful preparations not 



merely the fibrous strands which are inserted on the roots of the hooks 

 and move the latter, but also a tissue of very vesicular cells, which is 

 surrounded on all sides by smaller cells, and occupies especially the 

 hookless half. This does not consist, however, of a simple connected 

 mass, but is divided into four adjacent balls by the fibres and cortical 

 cells which insinuate themselves into it. The hooks themselves are 

 strongly developed (O015 mm. in length), and provided at the ends 

 with a sharply defined sickle-like claw, 0*006 mm. 



The longer the embryo lives freely, and the more the mantle 

 becomes inflated, the more do the em- 

 bedded granules disappear. In the same 

 way, the activity of the cilia decreases 

 and the motion becomes slower. At last 

 the embryo sinks to the ground. The 

 ciliary activity ceases, but the move- 

 ments of the hooks and the contractions 

 of the body may still be observed for 

 some time. In many cases, however, the 

 mantle has already been stripped off, so 

 that the embryo is now completely free, 

 and moves about with lively movements 

 of the hooks and of the body. It not un- 

 frequently happens that while the embryo 

 is escaping, only the external lamella of the mantle is rent, and in 



FIG. 388. Embryo of Bothrio- 

 cepJialus latus escaping from its 

 ciliated envelope. 



