274 



ZOOLOGY 



8BCT. 



endocoele becomes subsequently rounded ofi to form the internal 

 lining membrane of the pharynx, the cavity of which remains cut 

 ofi from the exterior by a partition, which is the outer part of the 

 wall of the endoccele, until the young worm is ready to leave the 

 egg. A short prolongation backwards from this cavity ends blindly 



in a mass composed 

 mainly of yolk, still con- 

 taining degenerate nuclei 

 and cells which have wan- 

 dered into it from the 

 blastoderm. Only at a 

 late stage, when the yolk 

 has become taken up, is 

 an arrangement of cells 

 recognisable in the form of 

 an intestinal epithelium 

 (endoderm) enclosing a 

 lumen (intestinal lumen). 

 An ectoderm likewise does 

 not appear as an embry- 

 onic layer, the epidermis 

 , only appearing late, and 



extending over the surface 

 evidently by modification 

 in situ of cells that have 

 become separated from the 

 main body of the blasto- 

 derm. The rudiments of 

 the brain are formed in 

 the blastoderm near the 

 wall of the endocoele, 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 : subsequently 

 these rudiments shift their position to the dorsal surface on 

 which the sacs form their permanent apertures piercing the 

 epidermis. There is no metamorphosis of any kind all the organs, 

 including 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 segmentation is the formation of a superficial layer of cells 



Fio. 224. Longitudinal section through the entire 

 egg of Temnocephala with the shell removed, 

 showing blastoderm with developing endoccele (en). 



