COLONIAL ANIMALS 103 
‘ p. 350). This, however, is not the only possibility, as will be seen 
by reference to fig. 1091, which represents a small part of a species 
of hydroid (Ag¢aophenza). In addition to the ordinary members of 
the colony, each provided with mouth and tentacles, there are 
two kinds of small mouthless individuals. One of these is in the 
form of a slender thread, which can be stretched out to some little 
distance, and is possessed of a thickened sticky tip. It acts as 
a food-catcher, ensnaring small animals to be swallowed and 
digested by its larger fel- 
lows for the benefit of the 
community. The other 
kind of mouthless indivi- 
dual is somewhat stouter, 
and richly provided at its 
free end with “ batteries” 
of stinging-cells, capable 
of dealing effectively with 
larger prey, or warding 
off the attacks of enemies. 
When these fighting in- 
dividuals ate called into #, Ordinary individual; 4, c, food-catchers, between them another 
action, the other members is seen capturing a crustacean larva, d, fighting individual; é, diges- 
: tive cavity of colony; _/, outer layer of body; g, horny investment. 
of the colony can be with- 
drawn into the little cups that surround their bases, being thus 
out of harm’s way. 
An extreme case of division of labour is presented by the free- 
swimming colonies of Hydroids known as Compound Jelly-Fish 
(Siphonophora), which have probably been evolved from simple 
medusz by a process of budding (see vol. i, p. 481). The shape of 
the colony depends upon the way in which this process has been 
effected. Sometimes the buds have arisen from the “umbrella” 
of the original medusa, or they may have grown from the walls of 
the mouth-bearing “handle”. The chief kinds of individual that 
have been thus produced are represented diagrammatically in fig. 
1092. The umbrella of the original medusa loses its function as 
a swimming organ and becomes a float, while (in the case repre- 
sented) the handle, of which part only is shown, carries a variety 
of members which contribute in various ways to the common weal 
of the community. Some are swimming-bells which, by alternately 
opening and closing, effect propulsion through the water. Others 
Fig. 1091.—Small part of a Colony of Agdaophenia, enlarged 
