266 W. WOODLAND. 
the base. As before, the triradiates are more or less sym- 
metrically disposed (far less so than in the oscular arrange- 
ment) with regard to the long axis of the sponge body, but 
their position is reversed [see text-fig. 8 (L) below, p. 269]. 
It is clear that, owing to the horizontal position of the 
diverticula, considerations as to the weight of the spicules 
being a possible factor in their disposition must be rejected. 
Again, owing to the absence of an osculum, invagination of 
the sponge-wall is also out of the question since there is no 
ready exit at hand from which the contained water may be 
expelled, the pores being too minute to allow of ready exit. 
The diverticulum, in fact, here resembles a water-cushion, 
and pressures tending to invaginate the wall are entirely 
resisted by the bulk of the contained water, and not by the 
paired arms of the triradiates. Since pressures incident on 
the spicules at right angles to the plane in which they lie are 
here non-existent, or at least ineffective as regards the pro- 
duction of the oscular mode of disposition—the body of the 
diverticulum being wholly uninvaginable—it is evident that 
the only pressures which can affect the triradiates are those 
which are incident laterally, for although uninvaginable the 
diverticulum is yet freely flexible about its base. The tri- 
radiates are affected by these pressures when they are situated 
more or less laterally with regard to the forces incident on 
the sponge (see text-fig. 5, B), and the triradiates are by them 
caused. to assume the non-oscular mode of disposition by 
adopting, as before, a position of equilibrium with regard to 
these incident forces. ‘Thus if a young spicule be initially 
disposed as in G, text-fig. 7, it will be evident that the 
tendency of lateral pressure (exerted on flexion of the sponge) 
from, say, the right side is to produce rotation of the spicule 
about its centre (the force impinging upon 8 long before it 
can reach A) the arms f and J rotating to the left and right 
respectively (H). If pressure be exerted on the left, rotation 
will occur in the opposite direction (I). Also, if the triradiate 
be initially disposed as in J, rotation will similarly take place. 
The larger the spicule grows the greater tendency will there 
