RFESOZOA. 



127 



APPENDIX TO SPONGES. MESOZOA. 



The title Mesozoa was applied by Van Beneden to some 

 very simple organisms which appear to occupy a very 

 humble position in the Metazoan series. The name sug- 

 gests a grade between the Protozoa and the Metazoa. 

 Haeckel called some of them Gastrreada;, regarding them 



A B 



Fir,. 36.— A. Young Dicyema. (Afler Wiutman.) 



B. Femalf OrthonecLid (Rhopaltn-a Giardii). 

 (After JULIN.) 



c Ectoderm ; en^ Inner endoderm cell with nucleus {>i) ; and 

 embryo {ciii)- 



as slight modifications of the hypothetical gastrula-like 

 ancestors of the Metazoa, while Hatschek, comparing them 

 to precociously reproductive planute speaks, of them as 

 Planuloida;. It is also possible that some of them may be 



