HYDRA AND CCELENTERATES. 133 
The hermit-crab, on the other hand, is probably pro- 
tected from enemies by the moss-like growth on its 
shell. The two animals seem to find this association 
mutually advantageous. The word commensalism is 
applied to animals living together in this manner. 
Campanularian Hydroids. These colonies may be 
easily collected, being found attached to logs or sea- 
weed, just at the level of low tide. The zooids may 
be stained with carmine solution for the purpose of 
making out the parts more easily. If there is time the 
pupils should note the following points: 
Examine a specimen, natural size, kept in formalin. 
How does it resemble a hydra? 
Do you find bilateral symmetry, either in the colony 
or the zooids ? 
Place a specimen in a watch-glass and examine it 
with a simple microscope. How many kinds of zooids 
do you see? 
In the feeding zooids can you see tentacles, hypo- 
stome, and a thickened body in a protecting, bell- 
shaped case? 
In other longer cases in the axils of branches do 
you see a number of round masses? 
These are really buds growing in the reproductive 
zooid. When fully mature, these masses float out at 
the top of the protecting case and swim away as small 
jelly-fishes. 
Examine both kinds of zooids, using a compound 
microscope. 
Summary of Drawings. (a2) A Campanularian 
colony of natural size. 
(0) A single feeding zooid slightly magnified. 
(c) A single reproductive zooid slightly magnified. 
Alternation of Generations. The jelly-fish which 
swim away from the reproductive zooid do not attach 
themselves to rocks, but remain free as long as they 
live. They are umbrella-shaped bodies, the mouth 
being at the end of the handle, their shape being like 
