270 W. WOODLAND. 
offer least resistance to the invagination of the wall, and all 
structures tend to place themselves in that position which 
enables them to offer least resistance to an incident force. 
Invagination of the sponge-wall, in fact, will act in the same 
manner as the lateral pressures above named. The protrusion 
of the monaxons on the sides of the sponge, or at the margin 
of the osculum or apex of a blind diverticulum, is an inevit- 
able result of their elongated form, disposition and place of 
origin, and the thinness of the body-wall. Their protrusion 
at the sides of the sponge is, perhaps, also, in large part, due 
to the possible reflexing of the wall-substance at the margin 
of the osculum in elongation of the sponge cylinder during 
growth. 
THE SECONDARY ForRMS AND OTHER FEATURES OF THE 
SPICULES IN CALCAREA. 
A few of the more conspicuous secondary features charac- 
terising the triradiate spicules found in the different groups 
of the Calcarea, and their possible causes must be brietly 
discussed. 
Spicules which develop, under undisturbed condi- 
tions, in the homogeneous substance of a wall of narrow 
breadth, assume an approximately ideal triradiate form, i. e. 
equi-angular, equi-radiate, and with the rays perfectly 
straight and “ finished.” Such are to be found in the majority 
of the non-vertical encrusting Clathrinid sponges. Spicules, 
on the other hand, which develop in the body-wall of the 
erect Leucusoleniide and Sycons, i.e. under disturbed 
conditions (since these sponges are constantly flexed to and 
fro by the motion of the surrounding water), are not so 
regular in form as those just instanced, the rays not being 
perfectly straight, and in many cases (above described for 
Sycons) deviating more or less from the equi-radiate type. 
In other words, the irregular “ sagittal ” form of spicule “ is 
correlated with the more erect growth of’ the Leucosoleniide 
