56 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



EXPLANATION OF FIGURES. 



All figures were taken from specimens of Metridium marginatum Milne-Edwards. 

 In the figures of transverse sections the dotted lines represent the axes of the mouths 

 projected upon the plane of section. All co* iplete mesenteries are drawn. The 

 incomplete mesenteries usually form more or less irregular groups in the primary 

 ectocoels. Each primary ectocoel containing incomplete mesenteries either has 

 these mesenteries represented in it or is marked with x ; any primary ectocoel not 

 designated in one or other of these ways contained no incomplete mesenteries. 



