472 



ON CONCRETIONS, SPICULES, ETC. 



[CH. 



which we have already described as occurring in certain sponge- 

 spicules) to the three superficial furrows between the adjacent 

 cells. This last is, as it were, an intermediate stage between the 

 simple rods and the complete formation of another concentric 

 sphere of latticed hexagons. Another possible case is when the 

 large and uniform vesicles of the outer protoplasm are mixed 

 with, or replaced by, much smaller vesicles, piled on one another 

 in more or less concentric layers ; in this case the radiating rods 



r^vAy 



Fig. 226. 



Fig. 227. A Nassellarian skeleton, Callimitra carolotae, Hkl. 



will no longer be straight, but will be bent into a zig-zag pattern, 

 with angles in three vertical planes, corresponding to the suc- 

 cessive contacts of the groups of cells around the axis (Fig. 226). 



Among a certain group called the Nassellaria, we find geome- 

 trical forms of peculiar simpUcity and beauty, — such for instance 

 as that which I have represented in Fig. 227. It is obvious at 

 a glance that this is such a skeleton as may have been formed 



