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 (Aglaophenia]. 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 are called into 

 action, the other members 

 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 

 medusae by a process of budding (see vol. i, p. 48 1 ). 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 Aglaophenia, enlarged 



a, Ordinary individual; b, c, food-catchers, between them another 

 is seen capturing a crustacean larva; d, fighting individual; e, diges- 

 tive cavity of colony; f, outer layer of body; g, horny investment. 



