130 
ANIMAL ACTIVITIES. 
mouth by the tentacles passes into the body-cavity, 
which is also a stomach. 
The hydra is really a bag, being hollow, even out in 
the tentacles. The food moves about inside of this 
bag and is dissolved there. The 
hard and useless parts of the food 
are pushed out through the mouth, 
which is the only opening into the 
bag. The nutriment is assimi¬ 
lated by the cells lining the bag 
or stomach-cavity. This lining 
is called the endoderm and its 
cells are very little differentiated 
from the ectoderm, or outside 
cells. 
Respiration. The cells of the 
hydra breathe independently, as 
do those of the sponge. 
Reproduction. If the hydra is 
divided into several pieces each 
piece reproduces the missing parts 
and becomes an active individual. A common method 
of reproduction is by the process of budding. The buds 
appear first as swellings along the sides of the parent 
hydra; they gradually develop a row of tentacles; a 
mouth breaks through into the common cavity of both 
parent and offspring; the base of the branch into which 
the bud has now grown constricts and finally breaks 
away entirely from the parent; and the hydra, free and 
independent, seeks its own place of feeding. 
But the hydra also reproduces by means of eggs. 
These appear as swellings on the side of the parent in 
much the same way as the buds just mentioned. Above 
the egg just below the fentacles another swelling 
appears. This contains the fertilizing or male cells 
{sperms'). The swelling containing the sperm-cells is 
called the testis. The ova as they mature float away 
into the water, as do also the sperms. A sperm swims 
F ig. i io. —Thread-cells 
{highly magnified ). 
