534 
SPONGES. 
and may be compared to the roots of a tree which divide into finer rootlets. The 
flagellated chambers form a convoluted cordon between the rootlets of the two 
systems. In the sea-kidney sponge of the Mediterranean the specialisation is 
carried to a still higher stage, eacli flagellated chamber being isolated from the 
rest, and having a slender canal leading to it, and one leading from it. 
The soft tissues permeated by canals require a supporting scaffolding or 
skeleton, and in nearly all sponges it is the function of certain cells in the 
mesoderm to secrete skeleton-material. A few sponges possess no skeleton 
whatever, excepting the gelatinous ground-substance; in some also the skeleton 
is mainly or entirely composed of foreign particles of sand, spicules of other 
sponges, skeletons of Radiolaria or Foraminifera. The vast majority form in 
the ground - substance a skeleton which is composed of spicules of silica, or 
carbonate of lime, or of horny fibres. The sponges whose skeleton is composed 
of calcium carbonate form a distinct class—the Calcarea. Of those which secrete 
a siliceous skeleton, the glass-sponges form a second class distinct from the rest 
of the siliceous sponges. All the rest, including by far the largest number, are 
included under a third class, the common sponges (Demospongia). 
The Calcareous Sponges,—C lass Calcarea. 
In this group the skeleton is formed of spicules of carbonate of lime, shaped 
a calcareous ascon SPONGE, Leucosolenia (magnified 4 times). 
like three-rayed stars, four-rayed stars, or needles. The triradiate occurs most 
frequently in its typical form, the three rays being equal, in one plane, and forming 
