STUDIES ON THE COMPARATIVE ANATOMY OP SPONGES. 329 
for the formation of secondary fibres to connect the primaries 
together. A reticulate skeleton must therefore be regarded 
as derived from a radiate one by the development of secondary 
fibres connecting the primaries. There can no longer be any 
doubt that the majority, at any rate of the so-called “ horny ” 
Sponges, are descended, probably along several lines, from the 
Halichondrina, by the gradual loss of spicules and the 
greater development of spongin in a reticulate skeleton. 
The occurrence of grains of sand in the skeleton fibres is 
not confined to the Ceratosa, for sometimes sand and 
spicules are simultaneously present in the fibres, as in 
Siphonochalina spiculosa (8). This replacement of 
spicules or of spongin by sand is no doubt of great advantage 
to the Sponge in saving material, and hence we not unfre- 
quently get Sponges whose skeleton is entirely arenaceous 
(e. g. Dy sidea). 
In Stelospongus f labellifor mis the skeleton fibres 
may sometimes be seen projecting freely from the surface of 
the Sponge (fig. 5, /'.), but this does not appear to be at all a 
constant character. It is difficult to understand how such a 
condition can have arisen ; perhaps it is in some degree com- 
parable to the projection of the spicules from the surface in 
very many siliceous Sponges. 
In the stalk the skeleton is more strongly developed than 
elsewhere, and hence it acquires a tougher and denser charac- 
ter than the remainder of the Sponge. 
The Canai. System. 
(a) The Pores. 
The inhalant apertures, or pores, are thickly scattered all 
over the depressed areas on the surface of the Sponge. Hence 
these areas might with some justice be termed pore-areas, 
in the sense defined by Ridley and Heudy in the Report 
on the “ Challenger ” Monaxonida (14). But there is an 
objection to the use of the term in this particular case in 
that these larger areas are themselves subdivided into a great 
