STUDIES IN SPICULE FORMATION. 265 
for the change from a relatively unstable to a relatively 
stable position, such as would obviously be the case here.! 
This supposition of a downward pressure (greater probably 
in its effect at one period of the growth of the spicule than 
another, except in the case of the apically-situated spicules 
of adult sponges) being brought to bear on the paired arms 
of the triradiates found in these sponges is confirmed when 
we observe that the depression towards the horizontal of 
these paired arms is the more marked the nearer the spicule 
is situated towards the apex of the (adult) sponge where 
flexion is greatest, the pressure on the sponge-wall having 
determined throughout the whole period of its 
activity the direction of growth of the apical actinoblast 
(see above, p. 236). 
There is a second mode of disposition of the triradiate 
spicules which is typically found in the blind (without an 
oscular aperture) free elongated diverticula of the genus 
Leucosolenia which at first sight appears anomalous and 
antagonistic to the explanations just provided for the case of 
the oscular arrangement of the spicules. This second mode 
of disposition, which we may term the non-oscular, was first 
pointed out by Minchin. In this arrangement the spicules 
are placed in an almost exactly opposite manner to that 
just described, i. e. the “ vertical ” or longitudinally-disposed 
ray tends to point towards the apex of the horizontal 
diverticulum, and the paired rays therefore tend to lie next 
1 The process of “righting” is actually to be seen in the case of many of 
the young triradiates, and conspicuously in that of Sycon ciliata, In this 
species, as before mentioned, one ray of the young triradiate is from the first 
considerably larger than its two companions, and this invariably becomes the 
“ posterior ” (vertically downward) ray of the full-grown spicule. If “ here- 
dity ” determined the vertically downward position of this larger ray, it might 
naturally be supposed that it would arise already so orientiated; but, on the 
contrary, the large ray is often found pointing as much as 80° from the down- 
ward vertical line. The change in position of the triradiate spicule which 
occurs as growth proceeds—as the spicule becomes larger—can, in my opinion 
only be attributed to the action of external causes, as above described; the 
greater weight of the larger ray is possibly also a factor. 
