38 ZOOLOGY 
Fic. 29.—Part of a section through 
Grantia labyrinthica, vertical to 
the margin and to the two surfaces 
of the wall of the cup. After 
Dendy. 
. Inhalent pore. 
Exhalent canal. 
Inhalent canal. 
Cavity of flagellate chamber. 
. Pore area. 
. Gastral skeleton, 
. Dermal skeleton. 
. Tubar skeleton. 
. Embryos. 
CONTR OV CO ND 
common form is somewhat flask 
shaped. At its free end are 
situated one or more slit-like 
openings, the oscula. The body 
is compressed from side to side, 
and its wall is pierced by 
numerous minute inhalent pores, 
which lead by a_ system of 
branching tubes into the central 
cavity. 
The substance of the sponge 
is composed of three layers 
the ectoderm, the endoderm, and 
between them the mesoderm. 
The ectoderm consists of flat- 
tened cells covering the outside 
of the sponge, and lining certain 
pits or depressions which are 
pushed into the substance of 
the sponge, and are termed in- 
tercanal spaces. The openings 
from the exterior into the inter- 
canal spaces are termed “pores” 
(1, Fig. 29). Several pores 
are usually grouped together,and 
form the pore area. The inter- 
canal spaces open on their inside 
by numerous apertures, called 
by Sollas “ prosopyles,” into the 
flagellate chambers (4, Fig. 29). 
These flagellate chambers are 
the most characteristic feature 
of the Sponges. They are lined 
by collared flagellate cells similar 
to those of the Choanoflagel- 
lata. Their flagella keep up 
a constant current of water, 
which passes in at the pores 
through the intercanal spaces and _ flagellate chambers, 
