352 WILSON. [Vou 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 Leucz/la 
ater, 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 Leuconta multiformis 
(transverse section, Pl. 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 Leuconta, 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 
