MESOZOA. 



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. 

 Hseckel called some of them Gastrseadae, regarding them 



A B 



FIG. 36. A. Young Dicyema. (After WHITMAN.) 



B. Female Orthonectid (Rhopalura Giardii}. 

 (After JULIN.) 



e. Ectoderm ; en, inner endoderm cell with nucleus (n) ; and 

 embryo (em). 



as slight modifications of the hypothetical gastrula-like 

 ancestors of the Metazoa, while Hatschek, comparing them 

 to precociously reproductive planulae speaks, of them as 

 Planuloidse. It is also possible that some of them may be 



