446 ON CONCRETIONS, SPICULES, ETC. [ch. 



on p. 314)^ we shall tend to obtain a spicule with two equal angles 

 and one unequal (Fig. 214, .4, C). In the last case, the two outer, 

 or superficial rays, will tend to be markedly curved. Again, the 

 equiangular condition will be departed from, and more or less 

 curvature will be imparted to the 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 state- 

 ment 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, for the interspace between our three contiguous equal 

 circles is an equilateral, curvihnear triangle; and even if our 

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

 but in the thickness of the intervening wall, then«we may recollect 

 (from p. 297) that the several partitions never actually meet at 

 sharp angles, but the angle of contact is always bridged over by 

 a small 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 those of Haeckel among others, 

 we shall find that all these various configurations are precisely 

 and completely illustrated. 



The tetrahedral, or rather tetractinellid, spicule needs no 

 explanation in detail (Fig. 214, D, E). For just as a triradiate 

 spicule corresponds to the case of three cells in mutual contact, 

 so does the four-rayed spicule to that of a solid aggregate of four 

 cells : these latter tending to meet one another in a tetrahedral 

 system, shewing four edges, at each of which four surfaces meet, 

 the edges being inchned to one another at equal angles of about 

 109°. And even in the case of a single layer, or superficial layer, 

 of cells, if the skeleton originate in connection with all the edges 

 of mutual contact, we shall, in complete and typical cases, have 

 a four- rayed spicule, of which one straight limb will correspond 

 to the line of junction between the three cells, and the other three 

 limbs (which will then be curved limbs) will correspond to the edges 

 where two cells meet one another on the surface of the system. 



