IX] OF THE SKELETON OF SPONGES 685 



growing, without marked change of form, by further deposition, or 

 crystalhsation, 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 

 estabhshed its characteristic but non-crystalline 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 little trio of cells; 

 and the little rods meet and fuse together while still very minute, 

 when the whole spicule is only about 200 ^^ ^ milhmetre long. 

 At this stage, it is interesting to learn that the spicule is non- 

 crystaUine; but the new accretions of calcareous matter are soon 

 deposited in crystaUine form. 



This observation threw difficulties in the way of former mechanical 

 theories of the conformation of the spicule, and was quite at variance 

 with Dreyer's theory, according to which the spicule was bound to 

 begin from a central nucleus coinciding with the meeting-place of 

 three contiguous cells, or rather the interspace between them. But 

 the difficulty is removed when we import the concept of adsorption; 

 for by this agency it is natural enough, or conceivable enough, that 

 deposition should go on at separate parts of a common system of 

 surfaces; and if the cells tend to meet one another by their interfaces 

 before these interfaces extend to the angles and so complete the 

 polygonal cell, it is again only natural that the spicule should first 

 arise in the form of separate and detached Hmbs or rays. 



Among the " tetractinelhd " sponges, whose spicules are com- 

 posed of amorphous silica or opal, all or most of the above-described 

 main types of spicule occur, and, as the name of the group implies, 

 the four-rayed, tetrahedral spicules are especially represented. 

 A somewhat frequent type of spicule is one in which one of the four 

 rays is greatly developed, and the other three constitute small 

 prongs diverging at equal angles from the main or axial ray. In 



