562 \ E. A. MINCHIN. 



metabolism, to explain why the porocyte secretes the gastral 

 ray. But none of all these factors of evolution can explain 

 why the presence of a young triradiate system in the vicinity 

 should stimulate the porocyte to resume its long-forgotten 

 functions, nor, above all, why it should not only secrete a 

 raonaxon spicule, but should go so far as to stick it on to the 

 triradiate system. The formation of the gastral ray is, indeed, 

 a most crucial test of the theories of spicular origin, and all 

 simply mechanical theories are at once, as it were, impaled 

 upon its relentless point.^ 



We have in the foregoing pages considered some very 

 different hypotheses in our search for the principle which 

 determined and modelled the fundamental types of the cal- 

 careous sponge spicule. We have examined the rival, and 

 often conflicting claims of crystallisation on the one hand, and 

 of the mechanical effects that might be supposed to follow 

 from the ultimate structure of protoplasm, from the dynamics 

 of the cell, or from the number and arrangement of the secret- 

 ing cells on the other hand. None of these theories, however, 

 stand the test of a thorough analysis, or can supply more than 

 a part of the explanation. All the evidence which we have at 

 our disposal drives us to seek the principle which guided and 

 directed the evolution of the spicular forms in a process of 



1 While the general method in which the spicules arise seems to me explica- 

 ble only by adaptation, there are characters in all of them which are certainly 

 diflScuIt to explain in this way. There are the peculiarities which separate and 

 distinguish the species, and which consist in small details of relative size, 

 length, curvature, or sharpness of the rays. A peculiarity of this kind in the 

 quadriradiates of Leucosolenia complicata, Mont. (= Ascandra com- 

 plicata, H., + A. pinus, H.), is worth mentioning in this connection, since 

 it seems to have escaped notice hitherto. In this species the gastral ray does 

 not arise from the junction of the three basal rays, but from the straight 

 posterior (unpaired) ray near its base — a fact which would be difficult to 

 reconcile with the theory of a primitive tetraxon spicule. Specific characters 

 of this kind are difficult to explain by any principle which requires that they 

 should be of utility to the organism, and they seem rather to have arisen by 

 the perpetuation in some unexplained way of a sport or variation not in itself 

 " useful " to the whole sponge. This is, however, no place to raise the much- 

 disputed question of the utility of specific characters. 



