340 EH. A. MINCHIN. 
to bring about simply the apposition of three mother-cells, 
and hence to produce three rays. It seems impossible to 
explain either by heredity or adaptation the definite rela- 
tions of the rays to crystalline symmetry; some other cause 
must be sought for the equiangular arrangement. Woodland 
has explained the equiangular condition as due to develop- 
ment of the spicule “under undisturbed conditions” (1905, 
p- 270), while the sagittal triradiates are supposed to be the 
result of development in disturbed conditions. I do not 
think this explanation can be accepted. Why should the 
spicules of a specimen of Clathrina coriacea, which lives 
between tide-marks and is subject to the rush of the tide 
four times in every twenty-five hours, roughly speaking, be 
supposed to develop under undisturbed conditions? Woodland 
has, moreover, confused together triradiates which are sagittal 
by deviations in the angles, and spicules which are sagittal 
simply by elongation of one ray, while remaining perfectly 
equiangular, as in Clathrina blanca and C. lacunosa. 
I find myself unable to attribute the wonderfully regular 
symmetry of the angles of the triradiates of Clathrinidee 
either to adaptation, as I did formerly, or to the conditions 
under which the spicule develops, as Woodland does. I 
can only refer the regularity of the angles between the rays 
to the operation of the physical laws resulting from the - 
crystalline properties of the material, which so modifies the 
growth of the rays relatively one to another, that it causes 
the three constituent elements of the compound spicular 
system to meet at the definite angles which are so constant a 
feature of these structures. 
Various objections naturally occur to this view, which I 
will discuss. The first is, how is the symmetry of the 
Leucosolenia-triradiate to be explained, where equal 
angles between the three rays are the greatest exception, if 
1 I do not feel confident that I have quite grasped Woodland’s meaning; 
the general trend of his argument seems to be that in the reticulate 
Clathrinide disturbances are balanced and do not preponderate in any one 
direction, as they do in the more erect Leucosoleniide. 
