io8 ANIMAL LIFE 



taking off but, what is of greater significance, their 

 increased vitality and productiveness. Needing but 

 little oxygen for their own consumption, they are able 

 to utilise any small surplus with great advantage, 

 as in growth ; and it is probably this help that 

 colour gives them. These reds and yellows, more 

 than the uncoloured parts, on this view, seize the 

 passing oxygen, store and then dispense it to the 

 tissues. 



The respiratory adaptations of Crustacea and 

 Molluscs. Among higher and more active creatures the 

 demand for oxygen is more insistent, and the irrigation 

 system becomes limited to particular regions, over which 

 a stream of water flows in and out. These regions 

 the gills are thin-walled folds of skin, within which 

 there flows a fluid which is no longer water, but blood, 

 thicker than water. Its channels lead to and away 

 from the gills, and the streams that fill them course 

 past the gill- walls, as they circulate. The sea- worms 

 bear a tuft of gill-plumes on every segment, and each 

 of the simpler sea-shrimps and pond-shrimps carries 

 a plume on its thigh. The very effort of movement 

 shakes the gill, brings it to fresh bodies of water, 

 and stimulates the circulation. The movements of 

 such creatures help to create the supply of oxygen 

 which they demand, and if food were easy to come 

 at and enemies few, there might be no need for 

 more elaboration than gill-plumes and gill-circulation. 

 Such, however, is notoriously not the case, and we 

 find in each kingdom that of the worm and that of the 



