IX 



OF THE SKELETON OF SPONGES 



681 



rays, wherever the cells of the system cease to be uniform in size, 

 and when the hexagonal symmetry of the system is lost accordingly. 

 Lastly, although we speak of the rays as meeting at certain definite 

 angles, this statement applies to their axes rather than to the rays 

 themselves. For if the triradiate spicule be developed in the 

 interspace between three juxtaposed cells it is obvious that its sides 

 will tend to be concave, because the space between three contiguous 

 equal circles is an equilateral, curvilinear triangle ; and even if our 



Fig. 313. Spicules of Granlia and other calcareous sponges. 

 After Haeckel. 



spicule be deposited, not in the space between our three cells, but 

 in the mere thickness of an intervening wall, then we may recollect 

 that the several partitions never actually meet at sharp angles, but 

 the angle of contact is always bridged over by an accumulation of 

 material (varying in amount according to its fluidity) whose boundary 

 takes the form of a circular arc, and which constitutes the " bourrelet " 

 of Plateau. In any sample of the triradiate spicules of Grantia, 

 or in any series of careful drawings, such as Haeckel's, we shall find 

 all these various configurations severally and completely illustrated. 



