GASTBMAD^ 



59 



dissimilar individuals. In the Dicijemidm no spermatozoa have yet 

 been discovered, but many egg-like germs, which apparently without 

 fertilisation develop as eggs within the axial cell. 



The course of development is as follows. The egg or the unicellular 

 germ divides into two unequal portions. The larger segmentation cell 

 which is thus produced (macromere) remains at first undivided, while 

 the smaller (micromere) divides repeatedly. The descendants of the 

 latter grow round the larger cell, finally completely surroimding it, 



Fig. 53.— Young Dioyema, after Whitman. 

 e, Ectoderm ; e%, endoderm cell, with nucleus 

 in) ; em, embryo. 



Fia. 64.- 



-Rhopalura Giardli, g 

 Julin. 



after 



and form the ectoderm, and, in the Orthonedidce, the muscular fibres as 

 well. The large cell remains undivided in the Dicyemidce and be- 

 comes the axial cell, while in the Orthonectidce it yields by division the 

 group of endodermal cells. 



Appendage : Triehoplax adhserens (Fig. 55). — This is a remark- 

 able animal discovered in the Graz marine aquarium, which presents 

 the appearance of a thin flat ciliated body like an Amceba, irregular 

 and varying in shape. It is composed of three layers — the lowest, 

 which adheres to the surface on which the animal rests, consists of 

 cylindrical cells, the uppermost of tesselated epithelium. The layer 



