SPONGES 
noflagellata. They take in and 
digest food and eject excre- 
ment from the area inclosed 
by the collar. The cilia (hairs 
by their constant movement 
create currents which keep the 
water in motion. Water, then, 
is taken through the pores into 
the first or incurrent canals; 
thence it is passed into the 
ciliated chambers, and thencé 
into the excurrent canals, and 
out through large passages 
terminating in large openings 
called oscula, or craters. The 
_ canal systems vary. In some 
species they become quite com- 
plex. 
Sponges vary greatly in 
shape, size, color, surface, rigid- 
ity, canal systems, and skel- 
eton. They are cake-shaped, 
tubular, digitate, palmate, cup- 
shaped, vase-shaped, cone- 
shaped, spherical, hemispheri- 
cal, pedunculate, etc., their 
shapes depending upon wheth- 
er their growth is uniform or 
is excessive in a horizontal or 
in a vertical direction. When 
they grow evenly in both direc- 
tions massive uniform shapes 
arise. If lateral growth pre- 
dominates, broad, low, and in- 
crusting shapes result. When 
there is an excess of vertical 
growth the forms are digitate ; 
~ 
103 
Vy " | i. 
Canal system of Sycon gelatinosum. Transverse 
section through the wall of a cylinder (parallel 
with the course of the canals), showing one in- 
current canal (IC) and one radial (R) throughout 
their length; sp, triradiate spicules; sp’, oxeate 
spicules of dermal cortex (dc); sp”, tetraradiate 
spicules of gastral cortex (gc); ec, ectoderm ; en, 
endoderm; pm, pore-membrane; pp, prosopyle; 
ap, apopyle; di, diaphragm; exc, excurrent pas- 
sage; PG, paragastric cavity; em, early embryo; 
em’, late embryo. (The arrows indicate the course 
of the water through the sponge.) 
