STUDIES ON THE COMPARATIVE ANATOMY OF SPONGES. 327 
size. The Sponge consists of a stout cylindrical stalk ter- 
minating below in a basal expansion for attachment to the 
substratum, and above in a broad, compressed, but thick frond. 
Along the upper margin of the frond is a single row of large 
oscula. 
The character of the surface of the frond is subject to con- 
siderable variations. On both sides it usually exhibits a 
number of prominent, branching, and anastomosing ridges, 
enclosing a number of concave depressions, as represented in 
the figure. The entire surface of the Sponge is more or less 
thickly encrusted with sand particles ; these are, however, 
more abundant over the ridges than in the depressions. The 
latter are also characterised by a minutely reticulate appearance 
due to the presence in them of the inhalant pore-areas, each of 
which bears a number of pores. The pores are enormously 
abundant all over the depressed portions of the surface, but 
appear to be absent from the ridges. 
The incrustation of sand already referred to gives to the 
surface of the Sponge a very hard, impenetrable character, and 
must form an admirable protection against the attacks of the 
numerous parasites to which Sponges are very subject. It 
functionally replaces the special dermal skeleton of spicules 
which exists in very many siliceous Sponges. 
The Skeleton. 
The skeleton (fig. 3) is composed of a rather irregular reti- 
culation of cylindrical horny fibres, branching and anasto- 
mosing freely. The fibres are rather slender, and the meshes 
between them are wide, so that in thin sections the skeleton 
scarcely appears at all. As in most horny and siliceous 
sponges, it is easy to distinguish between two sets of fibres, 
primary and secondary. The primary fibres (fig. 3 are 
long and rather stouter than the secondaries, measuring about 
O096 mm. in diameter. They radiate towards the surface of 
the Sponge, sometimes branching in their course, and end in 
the sandy incrustation. But they are most easily distin- 
guished by the presence in them of numerous foreigD bodies, 
