59 



disi. . .lar individuals. In the Dicyemidce no spermatozoa have yet 

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

 fertilisation develop as eggs within the axial cell. 



Th'-i 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 surrounding it, 



FIG. 53. Young Dicyema, after Whitman. 

 e, Ectoderm ; en, endoderm cell, with nucleus 

 (n) ; em, embryo. 



FIG. 54. Rhopalura Giardii, (j> , after 

 Julin. 



and form the ectoderm, and, in the Orthonectidce, 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 Amoeba, 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 



