MATERIALS FOR A MONOGRAPH OF THE ASCONS. 389 
attaches to the study of spicule-formation arises from the 
fact that in some cases, at least, we have before us, as it 
were, the meeting ground of inorganic and organic forces. 
In calcareous sponges the spicules are almost entirely com- 
posed of calcite, which is a definite crystalline substance, 
and ‘hence: subject to laws which result from its physical 
structure. On the other hand, the spicules arise in living 
cells, and the forms which they assume are undoubtedly 
correlated with the bionomics of the organism rather than 
with the physical properties of the skeletal material. The 
fitness of the triradiate spicules for supporting the body-wall 
is obvious to anyone acquainted with the structure and life 
conditions of the sponge. On the other hand, the monaxons 
may be reasonably supposed to form an efficient protection 
to the sponge as well as, probably, contributing something 
to its support; while the gastral rays, curving up towards the 
oscular opening, very probably tend to keep out parasites, as 
Dendy has suggested, and I am quite unable to agree with 
Woodland (1905, p. 239) that they are entirely function- 
less. Si 
Maas (1900 [1]) and Weinschenk (1905)! regard the 
spicule as the result of two activities, one organic-cellular, 
determining the form as a whole, the other inorganic-crystal- 
line, responsible for the molecular structure of the contents. 
That this is, in a general way, a correct statement of the 
facts of the case no one can doubt. The chief question is the 
exact limitation between the respective spheres of influence 
of the vital and physical forces, and, in particular, whether, 
and to what extent, the form of the spicule may be influenced 
‘by its physical properties, that is to say, its crystalline 
nature. 
In discussing this question we may begin by distinguishing 
1 «Tn every instant of the formation of a calcareous spicule, the deposition 
of the organically secreted calcite on the parts already present takes place 
according to purely inorganic laws of crystallisation, but there must always be 
present an organic element, which in its turn hinders the external inorganic 
form-production, and subordinates the external form to the conditions of the 
organism.’’— Weinschenk (1905, p. 584). 
