LOCOMOTION 
161 
which help the hydra to capture living water fleas, and the 
like. These stinging cells have darts which are automatically 
discharged when the ten¬ 
tacles come in contact 
with little animals. The 
darts stun the prey and 
render escape impossible. 
The tentacles surround 
the food and carry it to 
the mouth, which opens 
directly into the food 
cavity. The action of the 
tentacles in doing this 
work suggests the idea 
that each tentacle has 
some way of realizing the 
efforts of the others. 
We should keep in 
mind that in the Metazoa 
the united cells are in 
connection with one an¬ 
other through the cell 
walls. This is true even 
if we are not able to trace 
the connections with the 
microscope. In the higher 
animals we shall find that 
connections between cells 
are made by means of 
nerve cells. The develop¬ 
ment of a nervous system 
only carries out division of 
labor to a greater degree. 
140. Locomotion. — The 
Figure 148. — Fresh-Water Hydras. 
They are usually found attached to a 
water plant as in this drawing. The ex¬ 
panded hydra has a nearly full-grown 
hydra attached to one side and a small 
bud near the base. Both these young 
hydras were produced by the method of 
reproduction known as budding. The 
larger but very short hydra near the sur¬ 
face of the water is in a contracted con¬ 
dition and would be as long as the larger 
of the two expanded ones, if it were feed¬ 
ing. The small crustaceans shown in 
this drawing constitute the main food of 
hydra. How do they capture them? 
adult hydra is usually found 
attached as in Figure 148. In this condition the only move- 
