REPRODUCTION 
163 
ble skeleton of the animals eaten is cast out through the 
mouth. 
142. Respiration and Excretion. — By osmosis, oxygen is 
absorbed from the water by the cells of the ectoderm. The 
water that enters the mouth carries 
oxygen, and by osmosis it is absorbed 
by the cells of the endoderm. At the 
same time the carbon dioxide from the 
cells is thrown off into the water. 
143. Irritability. — The hydra is able 
to appreciate a variety of different 
kinds of stimuli such as jars, a moving 
animal or an enemy. It is able to con¬ 
tract, expand, and move the tentacles 
in such a way as to bring food to the 
mouth although it does not possess 
nerves or a brain. When a special 
study of the structure of the hydra is 
made, nerve cells are found which 
assist it in responding to stimuli. In 
the Protozoa there are no special nerve 
cells but the hydra shows the beginning 
of the formation of a nervous system. 
144. Reproduction. — The hydra re¬ 
produces both sexually and asexually. 
In sexual reproduction eggs and sperms 
are produced by the ectoderm cells. 
The sperm cells escape into the water 
and, like sperm cells of all other animals, have the power of 
locomotion. The fusion of the egg cell and a sperm cell 
starts growth which results in the division of the egg cell into 
many other cells. Hydras also reproduce asexually by bud¬ 
ding. The buds soon separate from the parent and begin 
an independent life. Like the developing sponge, the develop¬ 
ing hydra grows until it finally becomes a fully formed hydra. 
Figure 151. — Photomi¬ 
crograph of Hydra 
Bearing Eggs Illus¬ 
trating One Form of 
Reproduction. 
Reproduction by budding 
is shown in Figure 148. 
