STUDIES IN SPICULE FORMATION. 261 
To ensure that neither of these reactions of the sponge 
shall become excessive, i. e. detrimental, it is necessary that 
means of support shall be developed, in order to preserve to 
some extent the vertical position of the sponge, and to 
maintain the appropriate distension of the gastral cavity. 
A support to protect the sponge-wall from undue vertical 
swaying is evidently furnished by a vertically-disposed 
skeleton, and likewise to maintain distension of the gastral 
cavity, there is needed a skeleton disposed in a horizontal 
manner, since flexion in either direction is resisted by skeletal 
material, the length of which is placed at right angles to the 
direction of stress in the plane in which flexion occurs. 
Hence the sponge skeleton must, under these conditions, be 
TErxt-FIc. 4. 
: . 
O 
constituted of both vertical and horizontal elements. 
Both of these elements are supplied by the numerous tri- 
radiate spicules contained within the sponge-wall, for it 
inevitably follows from their conformation that if one ray be 
vertically disposed, then the two companion rays will lie in 
lines only deviating from the horizontal by an inclination of 
30°, and hence the three rays practically constitute two 
axes, respectively lying in the required vertical and horizontal 
directions. 
With regard to the two other forms of spicules—the 
monaxons and gastral rays—these probably do not in general 
exert a skeletal function, though the former lend considerable 
support to the oscular rim. The principal function of the 
1 The teleological form of the argument is merely adopted for brevity’s 
sake. 
