262 W. WOODLAND. 
monaxons is doubtless protective in nature,! preserving the 
sponge from the attacks of other organisms by covering its 
surface with a multitude of sharp spear-heads. The gastral 
rays, as before mentioned, are doubtless functionless. 
The triradiates can be affected in two ways by the pressures 
incident upon the sponge wall. If (a) the triradiates be 
situated in those portions of the wall which are in line with 
the direction of the incident force (in the plane of 
flexion of the sponge), then each individual spicule is acted 
upon by the incident pressure (transmitted through the 
gelatinous matrix) at right angles to the plane in 
which the three constituent rays lie, either from the side 
TExtT-FIc. 5. 
adjacent to the force or from the side opposite (text-fig. 5, A). 
If, on the other hand ({3), the triradiates be situated in those 
portions of the wall which are laterally placed with 
regard to the direction of the incident force, then 
each individual spicule is acted upon in the same plane as 
that in which it lies, i.e. laterally (B). As will shortly 
be shown—and as is, indeed, self-evident—pressures acting at 
right angles to the plane of the spicule have much more 
effect in determining the position of the spicule than pressures 
acting laterally in that plane, and, in consequence, when 
1 That is to say, the monaxons happen to possess this function. I do not 
wish to lend countenance to the common belief that every structural feature 
necessarily possesses a use or function. : 
