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, each flagellated chamber bring isolated from the 

 rest, and having a, slender canal leading to it, ami 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, — Class 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 



