ELISHA MITCHELL SCIENTIFIC SOCIETY. 37 



posed, for the colonies of calcareous sponges show plainly 

 that the budding individuals do not always develop oscula. 

 And on the other hand, there are certain indications in the 

 silicious sponges that in the adult oscula may be developed 

 almost anywhere. In spite of the difficulties, however, in 

 fixing upon the limits of the component individuals the 

 higher sponges are best regarded as colonies. Perhaps the 

 nearest approach made in other groups to the formation of 

 such colonies, in which the personality of the component 

 individual is so nearly lost, is found in corals like Maean- 

 drina, in which the united gastric cavities of the polyps 

 form continuous canals perforated at intervals by mouths. 



We therefore reach the conclusion that the higher sponges 

 (non-calcareous) have been derived from colony-producing, 

 symmetrical forms, in which the evaginations of the primi- 

 tive paragastric cavity had already taken the form of 

 efferent canals and flagellated chambers; that is, from forms 

 allied to the existing Leucons. /Vnd we further come to 

 the conclusion that the subdermal cavities and afferent 

 canals are homologous with the intercanals of Sycons, and 

 hence phylogenetically, at least, are infoldings of the ecto- 

 derm. The whole efferent system (canals and flagellated 

 chambers both), on the contrary, is homologous with the 

 same system in the calcareous sponges, and is endodermic. 



This conclusion as to the parts played by the germ layers 

 in producing the adult non-calcareous sponge, is the one 

 enunciated by Schulze in his classical paper on the 

 Plakinidae (p. 438). In this little family of silicious 

 sponges Schulze finds a genus, Plakina, the three species 

 of wdiich form links in a chain of increasing complexity, 

 showing quite as clearly as did the calcareous sponges that 

 the afferent system is derived from ectodermal infoldings, 

 and the efferent from endodermal outfoldings. 



The Plakinidae are Tetractinellids. The three species 

 of the genus Plakina are small encrusting sponges found in 



