STUDIES ON THE COMPARATIVE ANATOMY OE SPONGES. 331 
In Stelospongus flabelliformis each subdermal cavity 
(fig. 2, s. c.) is a hollow space corresponding in size and form 
to the pore-area which it underlies, and communicating with 
the exterior by means of the pores in its roof. The different 
subdermal cavities are separated from one another by anas- 
tomosing vertical walls of tissue constituting the bulk of the 
ectosome — represented as seen from above in fig. 4 ( cy .) and 
in section in fig. 2 (cy.) ; each one communicates below with a 
very much larger inhalant channel (fig. 2, i.l.). Thus each 
subdermal cavity receives the stream of water directly from 
the exterior through five or six distinct apertures in its roof, 
and passes it on through a single aperture in its floor into a 
relatively large inhalant channel. Just as a number of pores 
lead into one and the same subdermal cavity, so also a number 
of subdermal cavities lead into one and the same inhalant 
channel. 
(c) The Inhalant Canal System below the Sub- 
dermal Cavities. 
The large inhalant channels (fig. 2, i.l.) into which the 
subdermal cavities directly lead are comparable to the sub- 
cortical crypts described by Sollas (17) in the Tetractinellida, 
but it is needless to apply a special name to them in Stelo- 
spongus. They are merely the larger proximal portions of 
the inhalant canal system, commencing immediately beneath 
the ectosome and penetrating deep down into the choanosome. 
These larger channels lead into an irregular system of much 
smaller, more or less lacunar channels, whose ultimate rami- 
fications open into the flagellated chambers (fig. 6) ; and 
numerous flagellated chambers open out of one and the same 
inhalant lacuna. 
It must not be supposed that the inhalant canal system is 
always constant in arrangement ; the above description applies 
to what appears to be a fairly typical case, but there seems to 
be a good deal of variation, especially with regard to the 
subdermal cavities and the channels into which they lead. 
