260 W. WOODLAND. 
Tue Moves or DISPosiTtIoN OF THE SPICULES IN CALCARBA. 
To ensure due comprehension of the explanations about to 
be given in connection with the several modes of disposition 
which the elements of the sponge skeleton assume under 
different conditions of life, it will be necessary to first briefly 
consider the sponge organism in its relation to the environ- 
ment, and to this end we may select as a convenient form of 
sponge either of the two species of Sycon, the spicules of 
which have been already described. 
Sycons situated in shallow water which is often in motion, 
or planted upon rocks exposed to the action of falling waves, 
are in either case subjected to incident forces of considerable 
magnitude, and it will be readily understood that, were it 
not for the presence of the solid supporting structures con- 
tained within the sponge-wall, the organism could not attain 
to any considerable size, owing to the fragile nature of its 
semi-liquid gelatinous substance. Hence it follows that all 
the stresses to which the vertical sponge cylinder is subjected 
are borne by the contained spicules, and these inevitably 
react to the forces incident upon them. 
The elongated hollow cylinder of which the Sycon! consists 
can be affected in two ways by the motion of the surrounding 
medium; thus, being attached by the slender base, it can 
either (a) bend vertically as a whole (just as a tree is swayed 
by the wind), or (b) the wall of the cylinder can be invagi- 
nated upon itself, so tending to obliterate the gastral cavity 
(text-fig. 4). This latter reaction of the sponge is obviously 
but another phase of the former, since invagination of the 
wall is merely a flexion of one half of the sponge relative to 
the other; nevertheless, the two reactions must be distin- 
guished, since they constitute two separate factors in the 
disposition of the spicules. 
1 In Sycons the presence of the chambers interferes with the simple cylin- 
drical form of the sponge, although the remarks sufficiently well apply to the 
thin-walled oscular region. Many Clathrinide and Leucosoleniide would 
serve as better examples of a thin-walled flexible cylinder, 
