CELL AGGREGATION AND DIVISION OF LABOR 



99 



free in the water. Near the float is a group of swimming bells {nedo- 

 calyces) which resemble medusie and whose function it is to propel the 

 colony through the water by their alternate contraction and expansion. 

 At intervals below the swimming bells occur brads or covering scales, 

 feeding polj'ps ^yhich ingest the prey and digest it for the entire colony, 

 sensory polyps which in some species at least also serve as digestive 

 organs, tentades (defensive and offensive individuals) provided with 



Fig. 65. — Physalia, the Portuguese mau-of-war, drawn from live animal floating on 

 the surface of the sea. cr, crest; p, polyp; /)«, pneumatophore; /, tentacle. (From Parker 

 and Haswell's Textbook of Zoology, after Huxley.) 



nematocysts, and gonophores (reproductive zooids) wdth or without bells. 

 A first examination of a siphonophore might lead to the conclusion that 

 it is a complex individual with half a dozen kinds of organs. By a 

 careful study of sele(!tcd forms, however, and by means of a comparison 

 of these with such forms as Obelia and Hydractinia and other Hydrozoa, 

 it may be determined that each type of zooid which in a siphonophore 

 resembles and functions as an organ is really a much modified individual, 



