338 E, A. MINCHIN. 
If, however, we waive these objections and ‘assume that 
it is a universal rule for calcareous monaxon spicules to 
arise between two nuclei, we are still, it seems to me, very 
far from explaining the forms these spicules possess. ‘The 
“twin centres of secretive activity would only furnish us with 
‘a more or less elongated, bi-polar, intra-cellular structure, a 
type of spicule, in fact, such as is commonly found in siliceous 
sponges. We should still require some other factors to 
explain the projection outwards from the body wall, the fre- 
quent curvature, and above all the barb. Woodland’s theory 
supplies, as it seems to me, nothing more than the simple 
rod-like spicule, which would be required on any theory of 
evolution as the starting-point for further bionomic adapta- 
tion; the raw material, as it were, for the action of selection 
or of the natural method, whatever it may be, by which the 
structural characters of organisms are brought into harmony 
with their needs as determined by the conditions under which 
they live. 
To sum up the conclusions which I have endeavoured to 
establish in the foregoing paragraphs, it seems to me that 
the forms of the monaxon spicules are in no way explicable 
by the physical properties of the material, and only to avery 
limited extent, if at all, as the direct mechanical outcome of 
the conditions under which they develop. The monaxon— 
spicules owe the peculiarities of their form chiefly and, 
perhaps, entirely to their relations to the sponge-body, and 
are adapted to the needs of the organism. 
We may consider now the aggregate spicules; these, in 
calcareous sponges, always take the form of triradiate 
systems, to which a fourth ray may or may not be added.! 
Here we have two main types to consider, the perfectly 
equiangular triradiate systems found in Clathrinide, and 
the alate or sagittal spicules, with one unpaired and two 
paired angles found in Leucosoleniide. It. is sufficient 
1 As pointed out above, the large monaxon spicules of Clathrinid are. 
probably to be regarded, in some cases at least, as aggregate spicules, perhaps 
derived from:modification of triradiate systems. 
