CLASS HYDROZOA 
SEAWEED-LIKE ZOOPHYTES AND SMALL JELLYFISHES 
HE hydroids have been called the nurses of jellyfishes. From 
casual observation these two forms would not be associated 
together, for the shrub-like organisms, which so much resemble 
plants that they are often collected and preserved as seaweeds, 
suggest only vegetable life. Examined with a glass, however, 
they disclose their animal nature. Along the stems, arranged in 
various ways, are small cups, from which protrude the numerous 
moving tentacles of the little polyps living within them. 
Hydroids are colonies of associated animals liwing a communal 
life. The multitude of individuals composing the colony are 
invested with a horny covering, the perisarc, which in some genera 
assumes a tree-like form. Through these stems and branches 
runs a fleshy tube, a thread of animal substance, which connects 
in one living whole the zodids, or individuals of the community. 
There is division of labor, as in other communities: some of the 
zodids obtain the food for the colony, and have tentacles around 
their open mouths; others have no mouths, but reproduce the 
species, and at certain stages of development liberate swimming- 
bells, or small jellyfishes (medusee). 
The typical hydroid colony is attached by a kind of creeping 
stem from which arises a vertical axis, which gives off short 
lateral, alternate branches bearing zodids at their ends. There 
is often more complex branching. The zodids in certain genera 
(tubularians) are uncovered; in others (sertularians) they are 
incased in a glassy, cup-like, horny sheath. 
. Three kinds of zodids, polyps, or hydranths—as they are indis- 
criminately called—are attached to the stem. Those having an 
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