POSITION OF SPONGES 



165 



sponge. Finally, sponges deserve mention as factors in 

 human civilisation. 



General zoological position. — Sponges form the first 

 successful class of Metazoa. They illustrate the beginnings 

 of a *' body," and the beginnings of tissues. Along with 



B 



Fig. 81. — A. Young Dicyema.— 

 After Whitman. B. Female 

 Orthonectid (Rhopalura giar- 

 dii). — After Julin. 



e., Ectoderm ; en., inner endoderm cell 

 with nucleus [n.) ; and embryo [em.). 

 Note the segmentation and the 

 fibrillation supposed to be muscular. 



Fig. 82. — Salinella. — 

 After Frenzel. 



1. Longitudinal section — 



a., anterior ; p., pos- 

 terior. 



2. Transverse section. 



the Coelentera, they differ markedly from the triploblastic, 

 coelomate Metazoa, which do not retain the radial sym- 

 metry of the gastrula. In their germinal layers and in 

 their internal cavity they differ so much from Coelentera 

 and all other Metazoa, that they must be regarded as on 

 a by-road of evolution. This has been emphasised by 



