PHYLUM PLATYHELMINTHES 



270 



separated from the main body of the blastoderm. The rudiments 

 of the brain are formed in the blastoderm near the wall of the 

 endoccele, and thus have no connection with an ectoderm. The 

 excretory sacs and main vessels are formed from a small number 

 of large cells connected with the wall of the endoccele : subse- 

 quently these rudiments shift their position to the dorsal surface 

 on which the sacs form their 

 permanent apertures pierc- 

 ing the epidermis. There 

 is no metamorphosis of any 

 kind — all the organs, includ- 

 ing even the male part of 

 the reproductive apparatus, 

 being well advanced towards 

 full development before the 

 young animal leaves the 



egg- 



The egg of a Cestode is 

 similar in essential respects 

 to that of a Trematode : 

 there is a tough, chitinoid 

 membrane or egg-shell, 

 which encloses not only the 

 ovum but a number of yolk- 

 cells. The result of seg- 

 mentation is the formation 

 of a superficial layer of cells 

 (ectoderm) and a central 

 mass, all enclosed in a mem- 

 brane composed of a single 

 layer of cells thrown off 

 when the embryo escapes 

 from the egg. The ecto- 

 dermal cells become ciliated, 

 so far as is known, only 

 in Bothriocephalus ; in the 

 others they are thrown off 

 or ultimately absorbed with- 

 out developing cilia. The central mass of cells alone forms the 

 embryo. The embryo, while still consisting of a small number of 

 cells, develops a series of six chitinous hooks. These early changes 

 all take place in the majority of Cestodes while the egg is still in 

 the uterus of one of the most posterior of the proglottides of the 

 parent worm. When the proglottis in question becomes separated 

 off, and has passed out from the body of the final host, the eggs 

 are discharged. 



In order that development may proceed further, the embryo 



FSG. 2-20. — Longitudinal section through the entire 

 egg of Temnocepliala witli the shell re- 

 moved, showing blastoderm with doveloiJing 

 endocoele (eJi), 



