36 JOURNAL OF THE 



efferent canals) except in the one respect that in the Leucon 

 there is a single central cavity opening by a terminal 

 oscuhim, while in most silicious and horny sponges there 

 are several orcnla leading into as many spacious efferent 

 cavities. But here the disposition of the calcareous sponges 

 to form indubitable colonies helps us out, for if we com- 

 pare the silicious or horny sponge with a colony of Leu- 

 cons instead of with a single one, we find that its deriva- 

 tion from such simple symmetrical forms is made easy. 

 We must suppose the complex non-calcareous sponge to be 

 a colony, in which the limits of the individuals have been 

 lost or obscured by the increasing thickness of the walls. 

 This increasing thickness would finally result in a more or 

 less complete fusion of the members of a colony into an 

 undivided mass with oscula scattered over the surface. 

 Each of the main efferent canals of the non-calcareous 

 sponge is homologous with the paragastric cavity of a sin- 

 gle Leucon. Both the canal and its set of branches, though, 

 are extremely irregular, having completely lost the sym- 

 metry of the ancestral type. The flagellated chambers 

 still bear the same relation to the efferent canals as they 

 did in the Leucon; /. <:'. , they are simple diverticula of the 

 canals. The system of afferent canals is obviously homolo- 

 gous with the same system in the Leucons, bearing identi- 

 cally the same relation as in the latter group both to the 

 flagellated chambers and the efferent canals. The sub- 

 dermal cavities (which are only modified portions of the 

 afferent canal system), communicating with the exterior by 

 numerous pores, though a late acquisition, are found in cer- 

 tain Leucons; e. g.^ Eilhardia Schulzei (Polejaeff, PI. IX). 

 In many of the Non-calcarea the colonial nature of the 

 sponge is indicated by the presence of elevations (oscular 

 tubes or papillae), bearing oscula on their summits. But 

 the number of oscula is not always to be taken as indicating 

 the number of individuals of which the sponge is com- 



