164 CCELEN TER A TES. HYDRA-LIKE ANIMALS 
LABORATORY STUDY 
The living brown or green hydras can usually be found in the spring or 
fall in most fresh-water ponds. They are easily collected by gathering 
the floating leaves and overhanging grass that is immersed in the water. 
Place this collection in a glass jar in the laboratory. In a couple of days 
the hydras will have moved from the grass to the sides of the jar. They 
can be examined by a small magnifying glass in the jar or be transferred 
to a watch glass and observed under 
the low power of the microscope. 
Watch the hydra contract, when jarred 
or touched. Note that the tentacles 
become very short. Try feeding with 
a small bit of raw meat. Make out the 
transparent ectoderm and the darker 
endoderm. Are there any buds ? What 
happens to the buds when the parents 
contract ? 
145. Hydroids. — Hydroidsare 
marine, hydra-like animals which 
are united in groups forming a 
tree-like colony (Figures 153, 
156, a ). They are often mistaken 
for plants. When the young 
hydroid first begins to grow, it 
looks like the fresh-water hydra 
(Figure 156, k). 
As the hydroid grows, branches 
form and on the end of each 
branch, tentacles and a mouth 
appear. Each branch is able to capture food and, after it 
takes what it needs, the surplus is distributed to other parts. 
This is easily brought about, as a common digestive cavity 
connects all the branches. The hydroid is termed a colony 
because all the branches are united and help one another 
in getting enough food for the colony. 
Some of the hydroids form curious buds which develop 
into medusce (me-du'se). (See Figure 152, A .) As soon as the 
Figure 152. — Photomicro¬ 
graph of the Colonial Hy¬ 
droid Obelia with Medusae 
Forming at A. 
