WORMS AND CCELENTERATA 285 



coloured species in the sea — medusae, anemones, 

 sponges, etc. — and are one of its chief ornaments. 

 One other relative of the green polyp, the brown 

 polyp, is often found in our waters, and also the 

 fresh-water sponge, which is often found on pieces of 

 wood and branches at the bottom of ponds. 



Let us return to the animal we mentioned first, the 

 hydra, or green polyp. When we examine it with the 

 microscope, we find that its colour is due to very small 

 granules that pervade the animal. These granules are 

 independent organisms, algae, that live in the polyp, 

 without suffering any injury, or doing any to the hydra. 

 The joint living is beneficial to both organisms. The 

 algai have a pretty well-assured maintenance in the 

 coelenterate, while this in turn benefits by the oxygen 

 they secrete. Here we have a case of symbiosis — a 

 joint life of two organisms, based on mutual advantage. 



There are a large number of these symbioses. Certain 

 marine Crustacea, that live in snail-shells, always carry 

 on them anemones, ccelenterates that secrete a stinging 

 substance when they are touched that paralyse many 

 organisms. The sea -anemones have this advantage 

 from the transportation that it enables them to obtain 

 food more easily than they would do otherwise, since 

 they have no power of locomotion. On the other hand, 

 the crustaceans are protected by the anemone^ from 

 their enemies, because if one attempts to drag them 

 from their shell, the coelenterate plies its batteries and 

 the assailant has to withdraw. There is another 

 instance of symbiosis in the relation of ants and lice. 

 We generally find ants running about on plants with 



