336 BE. A. MINCHIN. 
between primary spicules, derived each from a single mother- 
cell, which may divide to form formative cells, and aggre- 
gate! spicules, derived originally from several mother-cells, 
and representing a fusion of two or more primary spicules 
ancestrally distinct from one another. Instances in the 
Leucosoleniide of primary spicules are the monaxons, and 
of aggregate spicules, the triradiates and quadriradiates. 
If we consider the monaxon spicules first it does not appear 
to me possible to correlate their form in any way with their 
physical, that is to say, their crystalline properties. To 
begin with, they always have unlike poles, which is a point 
of difference from any crystalline body, but which can be 
correlated with their situation in the body-wall, one end 
being embedded in the tissues, the other projecting free into 
the water. The small monaxons of L. complicata are 
straight, but in most cases the shaft is curved in such a way 
as to cause the free apex to project more vertically outwards 
into the water. The frequent presence of a distinct barb at 
the free distal end is a character which may be regarded as 
increasing the efficiency of the spicule as a protective weapon, 
and I would invite those who find such an explanation too 
teleological for their taste to put forward an alternative 
theory, and to explain the presence of the barb on mechanico- 
physical grounds. 
If crystalline properties fail to explain the forms of the 
monaxons, we may inquire what other causes can have pro- 
duced the results that are before us. Woodland (1905), if I 
understand him rightly, considers that the presence of two 
nuclei as two centres of cell-activity, between which the spicule 
arises, explains the monaxon form. But we do not find that 
all monaxon spicules arise between two nuclei. The monaxon 
spicules of calcareous sponges certainly appear to do so in- 
variably, but those of siliceous sponges, according to the 
1 T prefer this term, suggested by Woodland (1907, p. 56), to the term 
secondary spicules used by me formerly (1906). Woodland uses the term 
“*secondary ”’ spicules for those which are partly formed by the action of 
adventitious cells, such as the Lithistid desmas. 
