IX] OF THE SKELETON OF SPONGES 449 



what is the histological nature or " grade " of the vesicular structures 

 on which it depends. In soi;ne cases (apart from sponges), they 

 may be no more than the little alveoli of the intracellular proto- 

 plasmic network, and this would seem to be the case at least in 

 one known case, that of the protozoan Entosolenia aspera, in which, 

 within the vesicular protoplasm of the single cell, Mobius has 

 described tiny spicules in the shape of httle tetrahedra with 

 concave sides. It is probably also the case in the small beginnings 

 of the Echinoderm spicules, which are likewise intracellular, and 

 are of similar shape. In the case of our sponges we have many 

 varying conditions, which we need not attempt to examine in 

 detail. In some cases there is evidence for believing that the 

 spicule is formed at the boundaries of true cells or histological 

 units. But in the case of the larger triradiate or tetractinellid 

 spicules of the sponge-body, they far surpass in size the actual 

 "cells"; we find them lying, regularly and symmetrically 

 arranged, between the "pore-canals" or "cihated chambers," 

 and it is in conformity with the shape and arrangement of these 

 rounded or spheroidal structures that their shape is assumed. 



Again, it is not necessarily at variance with our hypothesis 

 to find that, in the adult sponge, the larger spicules may greatly 

 outgrow the bounds not only of actual cells but also of the 

 ciliated chambers, and may even appear to project freely from the 

 surface of th6 sponge. For we have already seen that the spicule 

 is capable of growing, without marked change of form, by further 

 deposition, or crystallisation, of layer upon layer of calcareous 

 molecules, even in an artificial solution; and we are entitled to 

 believe that the same process may be carried on in the tissues of 

 the sponge, without greatly altering the symmetry of the spicule, 

 long after it has established its characteristic form of a system of 

 slender trihedral or tetrahedral rays. 



Neither is it of great importance to our hypothesis whether 

 the rayed spicule necessarily arises as a single structure, or does 

 so from separate minute centres of aggregation. Minchin has 

 shewn that, in some cases at least, the latter is the case ; the 

 spicule begins, he tells us, as three tiny rods, separate from one 

 another, each developed in the interspace between two sister- 

 cells, which are themselves the results of the division of one of a 



T. G. 29 



