140 



ZOOLOO T. 



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

 tire functions being accomplished by the activity of the. 

 ectodermic cells and of the single axial cell. There is n» 

 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 Jfesozoa. 

 He places th& 

 branch, or sub- 

 kingdom, between, 

 the Protozoa and. 

 all the many-celled. 

 animals (Jfetazoa), 

 and includes the- 

 hypothetical Gas- 

 trcBades of Haeckel 

 in the branch. 

 "Wliile this position 

 may prove to be- 

 the correct one, we 

 should prefer, while 

 not overlooking the 

 resemblance of the 

 Dicyemidm to the 

 Infusoria, and even 

 the Gregarina?, tO' 

 wait for more light 

 on the development 

 of the parasitic 

 Platyhelminth 

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

 Dicyemidw, 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. 



g^ g. germigeTies ; 



nucleus of the axial cell ; b, the spherical germ of Dicye- 

 mella, ^ith its striated nucleus ; c, the same beginning 

 to undergo self-di^'ision ; d^ final stages of self-division 

 (mornla) ; « and f, inf usoriform einbryo ; A, genns of 

 the vermiform embryos of Dicyema typus ; i. gastmla 

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

 larvae of IHcyema typus, all highly magnified.— After E. 

 Van Beneden. 



