114 LIPE • OUTLINES OF GENERAL BIOLOGY 



and associated polymorphism — diiferent castes of individuals in 

 the colony working each in its own way, yet for the common good. 

 Thus in hundreds of different kinds of Alcyonarians and Penna- 

 tuHds there is dinior])hism, minute sij)honozooids keeping up a 

 current throughout the colony, while the larger autozooids look after 



Fig. 33. 



Diagr.^m showing the Division of Labour in a Siphonophore Colony, related 

 to the Portuguese Man-of-\Var. After Liing. i, An individual modified 

 into a float; i, an individual specialised for swimming; 3, an individual 

 showing the l)cginning of reproductive cells; .\, an individual modified 

 into a l)ract-likc protective sheath; 5, an individual modified into a long 

 tentacle, l>caring many stinging cells; 6, an ordinary nutritive, polyp-like 

 individual. 



nutrition and effect reproduction. In some of the hydroid colonies, 

 such as the Hydractinia which covers the borrowed whelk shells 

 tenanted by hermit-crabs, the div^ision of labour results in nutritive, 

 reproductive, st^isory, and jxThaps defensive tj'pes of individual. 

 In the free-swimming Siphono])hores, such as the Portuguese Man- 

 of-\Var, the jwlymorphism or division of labour in the colony 

 reaches its climax, for there may be as many as half a dozen different 

 kinds of individuals. This should be compared with the diversity 



