CLASS HYDROZOA 



SEAWEED-LIKE ZOOPHYTES AND SMALL JELLYFISHES 



THE 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 living 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 zooids, or individuals of the community. 

 There is division of labor, as in other communities : some of the 

 zooids 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 (medusas). 



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 zooids at their ends. There 

 is often more complex branching. The zooids in certain genera 

 (tubularians) are uncovered; in others (sertularians) they are 

 incased in a glassy, cup-like, horny sheath. 



Three kinds of zooids, polyps, or hydranths as they are indis- 

 criminately called .are attached to the stem. Those having an 



lid 



