156 
THE SIMPLER METAZOA 
with flagella which, by their movement, produce a current 
of water through the central cavity. The water enters 
through the holes in the sides (inhalent pores) and is forced 
out through the opening at the top pore (exhalent pores). 
The water contains food particles which the cells of the 
endoderm have the power to take in and digest. The food 
solution is passed to the other cells in the .sponge body by 
the process of osmosis. This is a process in which gases or 
liquids of unequal densities, separated by a plant .or animal 
membrane, tend to mix and become alike, the liquids or 
gases passing through the membrane. 
Thus the food digested is passed on and nourishes the cells 
of the middle and outer layers. The cells of the middle 
region form spicules (spic'uls) of lime (Figure 147) that pro¬ 
ject through the other layers and strengthen the whole body. 
The outer layer or ectoderm (ek'to-derm) serves as a protec¬ 
tive layer and with the help of the spicules gives definite 
shape to the body. 
LABORATORY STUDY 
The sponge which we ordinarily handle is simply the skeleton, and is 
easily kept from year to year. Examine several' kinds of sponge skele¬ 
tons and compare their shape, size, and the nature of the skeleton. How 
much water will the pores of the sponge hold ? Microscopic sections of 
Grantia are necessary if you are to make out the inhalent pores, the 
central cavity, and spicules. 
134. Reproduction. — At certain times of the year the 
sponge reproduces by means of two kinds of cells (eggs and 
sperms) developed in the middle layer. A sponge may 
develop both eggs and sperms, but usually develops 
only one kind at a time. Cells from the middle layer 
move in between cells of the endoderm and grow large and 
round. These are the eggs (female cells). Other cells move 
into the endoderm layer and divide into many small ciliated 
cells (the sperm or male cells). The sperms are set free and 
escape into the water of the central cavity and out from the 
