140 



ZOOLOGY. 



layer of cells, nor of any organs, all the animal and vegeta- 

 tive functions being accomplished by the activity of the 

 ectodermic cells and of the single axial cell. There is no 

 mesodermic cell or cells. On account of these characteris- 

 tics, Van Beneden 

 regards these or- 

 ganisms as forming 

 the type of a new- 

 branch of the ani- 

 mal kingdom, 

 which he distin- 

 guishes as Mesozoa. 

 He places the 

 branch, or sub- 

 kingdom, between 

 the Protozoa and 

 all the many-celled 

 animals (Metazoa), 

 and includes the 

 hypothetical Gas- 

 trceades of Haeckel 

 in the branch. 

 While this position 

 may prove to be 

 the correct one, we 

 should prefer, while 

 not overlooking the 

 resemblance of the 

 Dicyemidce to the 

 Infusoria, and even 

 the Gregarinas, to 

 wait for more light 

 on the development 

 of the parasitic 

 Platyhelminth 

 worms. It is not improbable, on the one hand, that the 

 Dicyemidce, retaining their parasitic life, are retrograde 

 forms, which have originated from some low Cestoid or 

 Nematoid worm, and bear the same relation to them, the 



Fig. 92. a, DicyemellaWagneri ; ff, g. germigenes ; n, 

 nucleus of the axial cell ; 6, the spherical germ of Dieye- 

 mella, with its striated nucleus ; c, the same beginning 

 to undergo self-division ; d, final stages of self-division 

 (morula) ; e and f, infusoriform embryo ; h, germs of 

 the vermiform embryos of Dicyema typus ; i, gastrula 

 of the same ; k, I, m, o, different stages of vermiform 

 larvae of Dicyema typus, all highly magnified. After E. 

 Van Beneden. 



