STUDIES IN SPICULE FORMATION. 267 
be for it to assume the position of equilibrium, and this posi- 
tion of equilibrium is evidently attained when the triradiate 
is in the position shown in either H or I, for when so placed, 
forces from neither side possess any tendency to produce 
rotation (the moments of the forces on (§ and A about the 
centre of the triradiate being then equal). Whether the posi- 
tion of equilibrium be attained by rotation of the spicule to 
the left or to the right evidently depends as to whether a 
pressure sufficient to produce rotation of the spicule into the 
position of equilibrium first arrives from the right or the left, 
when the spicule has attained a sufficient size to be so influ- 
enced by the pressures on the sponge. 
Trxt-Fic. 7. 
The spicule having attained the position of equilibrium it 
appears that the ray situated nearest the apex of the diverti- 
culum (§ and a in H and I) tends to lengthen somewhat, so 
giving the triradiate a sagittal appearance. The causes of 
this lengthening and the consequent tendency of the spicule 
to assume a more symmetrical position with regard to the long 
axis of the diverticulum are doubtless the same as those con- 
cerned in the lengthening of the posterior rays of the basal 
triradiates of Sycons (see p. 237 above), and the vertical dis- 
position of monaxons mentioned below. 
And now observe a striking confirmation of the above con- 
tention that the mode of disposition assumed by the triradi- 
ates depends, other things equal, on the presence or absence 
of an osculum. After the Leucosolenia diverticulum has 
