STUDIES IN SPICULE FORMATION. 263 
pressures are alternately incident upon the spicules in both 
these directions, only the effects of the former need be taken 
into account. 
Recognising these facts, it is now possible to inquire as to 
the causes which have brought about the several modes of 
disposition of the spicules in Calcarea. And first we will 
consider the mode of disposition found in those sponges 
which are cylindrical in form and possessed of a thin 
wall, which are mobile about a fixed base, which 
possess an oscular aperture at the distal free ex- 
tremity of the cylinder, and which are constantly 
flexed by the movements of the surrounding water 
(many Sycons, some Clathrinide and some Leucosoleniidz). 
This disposition, which we may term the oscular disposition, 
is best observed in the Homoceela, owing to the absence of 
diverticula of the body-wall. In all sponges characterised 
as above, the triradiates are situated in the sponge-wall in 
such a manner that in each spicule one ray points towards 
the base of the sponge (in erect forms vertically downwards), 
whilst the two companion rays necessarily le towards the 
apex (i.e. incline upwards in erect sponges at an angle of 30° 
to the horizontal), the whole spicule thus being symmetrically 
disposed with regard to the long axis of the sponge-body 
(p. 269, K). 
An oscular aperture being present, it is only necessary to 
consider the forces incident upon the spicules at right angles 
to the plane in which they lie (text-fig. 5, A). It is obvious 
that, were it not for the presence of the transversely-disposed 
rays of the triradiates, the sponge-wall would be invaginated 
to a smaller or greater extent whenever the sponge was 
affected by motion of the surrounding water, since the vertical 
element of the skeleton (vertical rays of triradiates and 
vertical monaxons) is not adapted to resist flexion in this 
direction. Hence invagination of the thin wall is resisted 
by the two upper arms of the triradiates, i.e. the portion of 
the sponge-wall adjacent to the paired arms of each triradiate 
tends to “bulge” through the space subtended by them, 
voL, 49, PART 2,—NEW SERIES, 19 
