352 WILSON. [Vol. IX. 



arrangement naturally having been increased by the folding of 

 the wall of the paragastric cavity. 



The increasing complexity in the Leucon family is brought 

 about by the ramification of the primitively simple efferent 

 canals, the radial tubes growing shorter and becoming in the 

 most complicated types spheroidal chambers quite like the 

 flagellated chambers of the non-calcareous sponges. In Lciicilla 

 titer, for instance, of which part of a transverse section is given 

 in PL XXV, Fig. 3, the efferent canals exhibit branching of a 

 simple character. But in such a form as Lcuconia muliiformis 

 (transverse section, PI. XXV, Fig. 4), the ramification of the 

 efferent canals becomes exceedingly complex, and the radial 

 tubes here appear as spheroidal flagellated chambers. The 

 intercanals (or afferent canals, as they are called in all sponges 

 but the Sycons) follow the efferent canals in all their windings, 

 bringing water from the surface pores to the pores in the walls 

 of the flagellated chambers. 



The chief conclusions to be drawn from this anatomical 

 comparison of the various forms of Sycons and Leucons are, 

 that the afferent canals of Leucons are homologous with the 

 intercanals of Sycons and are lined with ectoderm ; that the 

 flagellated chambers are homologous with the radial tubes ; 

 that increasing complexity is brought about by the ramification 

 (or folding of the wall) of the efferent canals. 



The canal system of a complicated Leucon, like Leuconia, is 

 essentially like that of a common silicious or horny sponge 

 (having flagellated chambers, afferent and efferent canals), ex- 

 cept in the one respect that in the Leucon there is a single 

 central cavity opening by a terminal osculum, while in most 

 silicious and horny sponges there are several oscula leading 

 into as many spacious efferent cavities. But here the dis- 

 position of the calcareous sponge to form indubitable colonies 

 helps us out, for if we compare the silicious or horny sponge 

 with a colony of Leucons instead of with a single one, we 

 find that its derivation from such simple symmetrical forms 

 is made easy. Robbed of its details, a silicious sponge of 

 the character of Esperella, Tedania, or Tedanione, exhibits a 

 structure illustrated by the diagram of an hypothetical silicious 



