THE SPONGES 3509 



channels. Each of the flagellated chambers in the walls of the folds communicates 

 with the in-current spaces through several pores, and opens into the out-current 

 spaces by one large pore, the currents of water passing out by the central oscule. 



The fine toilet sponge possesses a more developed canal system. The in-current 

 and out-current parts of the water-bearing system are more definitely "canalized," 

 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 sys- 

 tems. In the sea kidney sponge of the Mediterranean the specialization 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. 



A CALCAREOUS ASCON SPONGE, Leucosolenia (magnified 4 times). 



The soft tissues permeated by canals require a supporting scaffolding or skele- 

 ton, 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 com- 

 posed 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). 



