132 ANIMAL ACTIVITIES. 
It is often found attached to snail-shells inhabited by 
hermit-crabs. The individuals live in a colony and 
vary according to the work each has todo. They all 
grow together at the base, and so are connected by 
living tissue through which digested food may be passed 
from one to another. 
Here one individual (a) does the eating for the colony 
with the help of others like himself; another (4) attends 
entirely to the work of reproduction; while still another 
(c) protects the colony from enemies. These hydra- 
like individuals, which are not strictly individuals 
because they do not lead an independent existence, 
are called zoords. Such an assembly of zooids is called 
Fic. 112.—Hydractinia (magnified). After Agassiz. A, male colony; 
B, female colony; a, feeding zooid; ¢, tentacles; 4, reproductive 
zooid; ¢, protective zooid; d, e, 7, g, 4, 2, stages of jelly-fish. 
acolony. Again we notice the setting apart or differ- 
entiating of zooids to perform each its own work. 
At the base of the zooids of Hydractinia a hard skele- 
ton is built, which resembles coral in its nature. 
Commensalism. As stated above, the Hydractinia 
is commonly found on shells inhabited by hermit-crabs. 
The Hydractinia colony is thus carried about and 
brought constantly into contact with a new food-supply. 
