HYDROZOA 



21 



waves, or the transporting, by the same means, of the debris 

 to which they are attached in later life. 



Symbiosis is exemplified by Hyd'ra vir'idis, or the green 

 Hydra, the color probably being due to the presence of small 

 green algce. 



Another species found in Russia, Polypd'dium hydrifor'me, 

 of which little is known, is parasitic on sturgeon eggs. 1 



A Hydroid Colony (Fig. 11). Suppose a hydra-like animal 

 to bud and branch until it looked like a tiny bushy shrub. This 

 will give you some idea of these plant-like hydroids. These 

 hydra-like animals, or polyps, are connected by a system of 

 tubes, the common stem or axis bearing many individual zooids. 



F\g.l2.Obe'liaflabella'ta. (Hincks.) Fig. 13. Obe'lia commissura'lis. 



Obelia (Figs. 12, 13) is a good representative of such colonies. 

 The axis is made up of a creeping horizontal portion and of 

 vertical axes. The short, alternate, lateral branches of these 

 axes bear zooids at their extremities, or, again branching, the 

 polyps or zooids are borne on the second set of branches. When 

 these zooids are immature, they are little, club-shaped enlarge- 

 ments. When mature, the polyps are surrounded proximally by 

 a little glassy, protective cup, the hydrotheca, and distally bear 

 about a score of tentacles. These are the nutritive zooids, 

 for division of labor is found here. The tentacle-bearing indi- 

 viduals procure the food, and since the tubes are all hollow 



1 Hertwig's " Manual of Zoology," Kingsley, p. 241. 



