450 



ON CONCRETIONS, SPICULES, ETC. 



[CH. 



little trio of cells ; and the little rods meet and fuse together while 

 still very minute, when the whole spicule is only about ^^ of a 

 millimetre long. At this stage, it is interesting to learn that the 

 spicule is non-crystalline ; but the new accretions of calcareous 

 matter are soon deposited in crystalline form. 



This observation threw considerable 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 the 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 the process of 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 conceivable and natural that the spicule should 

 first arise in the form of separate and detached limbs or rays. 



Among the tetractinellid sponges, 

 whose spicules are composed of amor- 

 phous 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 

 V J t, equal angles from the main or axial 



V^ I ^ j ray. In all probability, as Dreyer 



^■^ ^ suggests, we have here had to do with 



■ ^ M a group of four vesicles, of which 



I I three were large and co-equal, while a 



" fourth and very much smaller one lay 



Pig. 215. Spicules of tetracti- above and between the other three, 

 nellid sponges (after Sollas). x . • ^ i i-i 



, . J f In certain cases where we nave nke- 



a-e, anatnaenes; a-j, pro- 



triaenes. wise one large and three much smaller 



