354 WILSON. [\'0L. IX. 



which in these sponges send a special little canal to each 

 chamber (see Schulze's figure of Euspongia. Zcit. f. U'iss. 

 Zoo!., Bd. XXXII, Taf. XXXVI, Fig. 2). The difference 

 between the two canal systems is easily explained, that of 

 Euspongia being derived from the type shown in Fig. 5, by the 

 pushing out of minute diverticula from both afferent and 

 efferent canals. 



Having now obtained a generalised idea of a complex non- 

 calcareous sponge, it will be found a simple matter to compare 

 such a form with a Leucon colony, of which I give a surface 

 figure, taken from Vosmaer, in PL XXV, Fig. 6. The structure 

 of the silicious sponge is readily understood if we suppose it to 

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

 lost or obscured by the increasing thickness of the walls of 

 adjacent individuals. 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 silicious 

 sponge is homologous with the paragastric cavity of a single 

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

 extremely irregular, having completely lost the symmetry of 

 the ancestral type. The flagellated chambers, however, still 

 bear the same relation to the efferent canals as they did in the 

 Leucon, i.e. they are simple diverticula of the canal wall. The 

 system of afferent canals is obviously homologous with the 

 same system in the Leucons, bearing identically the same 

 relation as in the latter group, both to the flagellated chambers 

 and the efferent canals. The subdermal chambers, communi- 

 cating with the exterior by numerous pores, though a late 

 acquisition, are found in certain 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 sphere is composed, for the colonies of 

 calcareous sponges show plainly that the budding individuals 

 do not always develop oscula. And on the other hand there 



