IX] 



OF THE SKELETON OF SPONGES 



445 



the bodies are unequal in size, the partitions are curved, and 

 combine to form other and less regular polygons. It is plain, 

 accordingly, that in any flattened or stratified assemblage of such 

 cells, a solidified skeletal deposit which originates or accumulates 

 either between the cells or within the thickness of their mutual 

 partitions, will tend to take the form of triradiate bodies, whose 

 rays (in a typical case) will be set at equal angles of ] 20° (Fig. 214. F). 

 And this latter condition of equality will be open to modification 



Fig. 214. Spicules of Grantia and other calcareous sponges. 

 (After Haeckel.) 



in various ways. It will be modified by any inequality in the 

 specific tensions of adjacent cells ; as a special case, it will be apt 

 to be greatly modified at the surface of the system, where a spicule 

 happens to be formed in a plane perpendicular to the cell-layer, 

 so that one of its three rays hes between two adjacent cells and 

 the other two are associated with the surface of contact between 

 the cells and the surrounding medium ; in such a case (as in the 

 cases considered in connection with the forms of the cells themselves 



