STRUCTURE AND LIFE-HISTORY OF A SPONGE. 27 
sometimes it is longitudinal, and then produces two similar flagell- 
ated cells, instead of a flagellated and an amoebiform cell. It is 
by longitudinal fission that the e’ndoderm grows in extent, corres- 
pondingly with the growth of the surface it covers. 
We will now resume our description of the structure of the 
sponge. 
The Skeleton . — The soft tissues of the sponge require some kind 
of support, and this is afforded to them by the hard parts, or 
skeleton. This consists of needle-shaped, tri-radiate, and quadri- 
radiate spicules, disposed in a definite manner. The proper wall 
of the stomach is furnished with three and four-rayed spicules, 
three of the rays of the quadri-radiate, and all those of the tri- 
radiate spicules, lying in the substance of the wall, parallel to its 
surface, while the fourth ray of the quadri-radiate spicules projects 
with a gentle upward curve into the gastric cavity, carrying the 
gastral membrane with it. 
The radial tubes are furnished with tri-radiate spicules, arranged 
in successive, concentric, or transverse rows. The form of each 
spicule is such that two of its rays, those including the largest 
angle, may be taken to form a pair, the third ray being therefore 
‘ odd j ’ and they are so arranged that the paired rays lie concen- 
trically, while the odd ray lies longitudinally, in the wall of the 
tube. The paired rays, which form the basal or proximal row of 
each radial tube, lie back to back, as it were, with the spicules of 
the stomach wall, which is thus doubly strengthened. 
The outer ends of the radial tubes are furnished, in addition, 
with colossal fusiform spicules, each often 3 mm. long ; these are 
embedded at one end in the tissue of the tube, and at the other 
project freely beyond it ; about the base of each colossal spicule 
there is usually a pencil of similar, but much smaller spicules, and 
a few large grapnel-like spicules are also present. The spicules, 
large and small, thus projecting from the end of the tube in the 
form of a brush or pencil, give to the surface of the sponge the 
