264. W. WOODLAND. 
when flexion of the sponge occurs (see text-fig. 6, F, below), 
and the triradiates being numerous and irregularly distributed 
(i.e. not arranged in vertical rows), invagination of the thin 
sponge-wall is almost entirely prevented. This resistance 
offered by the paired rays of each individual triradiate is, as 
already implied, the means whereby the symmetrical dis- 
position of the spicule is brought about. For if we suppose 
that a triradiate is not symmetrically placed with regard to 
the long axis of the sponge-body (as in C or BE), then it will 
be evident that on flexion of the sponge next occurring in the 
appropriate direction, the spicule will at once be “righted,” 
for the arm that is more inclined towards the vertical will be 
influenced by the pressure on the sponge wall sooner than 
Text-Fic. 6. 
F 
the lower arm, and hence the spicule will be rotated 
about its centre until the two arms are similarly 
disposed with respect to the incident force (D). This process: 
is represented in the above diagram. If a cylinder of 
paper be taken, and one upper side pushed inwardly, it 
can easily be understood that it would tend to “right” an 
asymmetrically-disposed forked structure, between the arms 
of which the invagination occurred (F). Even if the young 
triradiate be so initially placed that the vertical ray points 
apically (towards the osculum), such a symmetrical position 
would not be maintained, owing to the flexion of the sponge 
not always taking place in an exactly vertical plane (speaking 
of vertical sponges) ; and, moreover, if the weight of the 
spicule be a factor in its disposition, there is still more reason 
