5^4 



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 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). 



t The Calcareous Sponges, — Class Calcarea. 

 In this group the skeleton is formed of spicules of carbonate of lime, shaped 



a calcareous ascon sponge, LeucosoUnia (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 



