THE CCELENTERATES 



37 



colony is fitted for a free-swimming existence. What cor- 

 responds ordinarily to the attached stalk in other forms 

 terminates in a bladder-like expansion, distended with 

 gas, that serves as a float. From it are suspended individ- 

 uals resembling great stream- 

 ers sometimes many feet in 

 length, without mouths, but 

 loaded with nettle-cells that 

 enable them to capture the 

 food, which is conveyed to the 

 second type, the nutritive 

 polyps. Each of these is a 

 simple tube bearing a mouth, 

 and within them the food is 

 digested and distributed by 

 means of a branching gastric 

 cavity extending throughout 

 the entire colony. Then there 

 are individuals like mouthless 

 jelly-fishes which bear the 

 eggs and care for the perpet- 

 uation of the colony ; and be- 

 sides these there may be some 

 whose duty it is to defend the 

 rest, and others whose active 

 swimming movements, to- 

 gether with the wind, drive 

 the colony about. Thus uni- 

 ted, sharing the food supply 

 and working for the general welfare of all, the members of 

 this colony live in greater security and with less effort than 

 if, as separate individuals, each was fighting the battles of 

 life alone. 



38. Scyphozoa, The greater number of the larger and 

 more conspicuous jelly-fishes are included under this term. 

 In general shape and locomotion they resemble those of the 



PIG. 20. A colonial jelly-fish (Physalia). 

 From Nature. 



