98 



I'RINCIPLES OF ANIMAL HIOLOGY 



phistn, meaning that within the same species individuals may have 

 different forms. 



The corals are fine examples of colonj^ formation and their skeletons 

 represent the results of the combined lime-secreting activities of the 

 colon}' continuing through a considerable period of time. Most corals 

 do not show tliffercntiation into two or more types of individual but such 

 differentiation occurs in Hydrocorallinaj. 



pneumatophore 



Fig. 04. — Diagram of a siphonophore colony coniijosed of six hinds of individuals. {Modi- 



^ed from Flcinchmann.) 



Colony Formation in Siphonophora. — The most remarkable examples 

 of colony formation and polymorphism accomi^anied by division of 

 labor occur among the Siphonophora, one of the orders of Hydrozoa. 

 These are free-swimming colonies of great complexity. Each sipho- 

 nophore colony (Fig. 64) consists of a common tulje of ca^nosarc which 

 bears at one end a pnewnatophore or float and along its lengtli zooids of 

 various forms. The float is the expanded end of the cocnosarcal tube. 

 It generally contains gas and serves to support the colony which hangs 



