278 ZOOLOGY 



have the habit of forming colonies by ])udding. In all cases 

 the buds arise from a definite part of the parent body and de- 

 velop into a definite form, often exactly like that produced from 

 the egg. When the buds remain attached to the parent, a 

 compound individual or colony is produced. These colonies 

 differ greatly in form. Thus among hydroids we have colonies 

 which jiroduce runners, from which alone, and not from other 

 hydranths, new hydranths arise. In another case (Obelia), 



Fig. 268. — Atoll in Fiji Lslands (Nariuku Le\ai). The large circle of white 

 made by breakers indicates the position of the coral reef. A small bit of 

 land still remains in the interior lagoon. Photo, by Dr. W. McM. Wood- 

 worth. From A. Agassiz, "Coral Reefs of Fiji." 



one hydroid buds from the side of another and rises beyond it, 

 continuing the main stem of the colony. Since its descendants 

 do the same, the stalk is made up of successive generations of 

 hydranths. Sometimes the hydranths are placed close to- 

 gether and opposite, like the leaves of arbor vitiB (Sertularia, 

 Fig. 258). Again, there maj' be a main stalk composed of one 

 h^'dranth and a series of lateral branches in one plane, making 

 a fan-like arrangement of the colony. Or the lateral branches 

 may rise in any plane, producing a bushy colony. The va- 

 riety in the form of colony possessed even by a single species 

 adds to the diversity of hydroids. 



In any colony a division of labor may occur among the con- 



