164 



GENERAL ZOOLOGY 



this class. This hydroid always grows as a branching 

 colony (Fig. 67). All individuals are intimately associ- 

 ated with each other and the food taken by one animal 

 may pass down the hollow stalk to nourish other members 

 of the colony. Such cooperative activity between different 



FIG. 67. Obelia, a colonial hydrozoan. A, a small colony somewhat enlarged; 

 B, portion of same greatly enlarged; 6, medusa buds; c, coenosarc, or soft flesh; 

 e, old branch from which an individual has broken off; g, gonangium, or repro- 

 ductive individual ; h, a mature hydranth ; or feeding individual ; j, joints to permit 

 bending of hydranth on its stem; ra, young medusa swimming out of gonangium; 

 p, perisarc; y, young hydranth. 



individuals makes division of labor possible and in Obelia 

 differentiation of functions has gone so far that some mem- 

 bers of the colony carry on reproduction and cannot feed, 

 others only feed and cannot reproduce. There are, then, 



