THE CQLENTERATES 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- Fie. 20.—A colonial jelly-fish (Physalia). 
2 From Nature. 
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 
