IX] 



OF THE SKELETON OF SPONGES 



451 



rays, the latter are recurved, as in Fig. 215. This type, save for 

 the constancy of the number of rays, and the Umitation of the 

 terminal ones to three, and save also for the more important 

 difference that they occur only at one and not at both ends of 

 the long axis, is similar to the type of spicule illustrated in 

 Fig. 213, which we have explained as being probably developed 

 within an oval cell, by whose walls its branches have been con- 

 formed to geodetic curves. But it is much more probable that 

 we have here to do with a spicule developed in the midst of a 

 group of three coequal and more or less elongated or cylindrical 

 cells or vesicles, the long axial ray corresponding to their common 



2/^ 



9 



Fig. 21(i. Various holothurian spicules. (After Theel.) 



line of contact, and the three short rays having each lain in the 

 surface furrow between two out of the three adjacent cells. 



Just as in the case of the little curved or S-shaped spicules, 

 formed apparently within the bounds of a single cell, so also in 

 the case of the larger tetractinellid and analogous types do we 

 find among the Holothuroidea the same configurations reproduced 

 as we have dealt with in the sponges. The holothurian spicules 

 are a little less neatly formed, a little rougher, than the sponge- 

 spicules ; and certain forms occur among the former group which 

 do not present themselves among the latter; but for the most 

 part a community of type is obvious and striking (Fig. 216). 



A curious and, physically speaking, strictly analogous forma- 

 tion to the tetrahedral spicules of the sponges is found in the 



29—2 



