160 THE STORY OF LIFE IN THE SEAS. 



These cases of animals bearing on their bodies 

 other creatures which are not in the strictest 

 sense of the word parasites, are but instances of 

 a phenomenon that is very widely spread among 

 marine organisms. There are many cases, how- 

 ever, in which plants and inorganic foreign bodies 

 play a very important part in the economy of 

 animals. 



In our chapter on the free-swimming organisms 

 of the ocean, mention has been made of the deli- 

 cate and beautiful creatures called Eadiolarians. 

 Many years ago it was discovered that each of 

 these animals bears in its protoplasm a number 

 of little cells, which from their colour received 

 the name of "the yellow cells." It was clear 

 from observation and experiment that they were 

 neither organs nor products of the Radiolarian, 

 but independent organisms belonging to the 

 Vegetable Kingdom. 



More recently cells similar to these have been 

 found in many of the Corals, in Worms, and 

 other animals, and there can be no doubt now 

 that when present they perform very important 

 physiological functions which materially assist 

 their host in its growth and development. 



So numerous are these " yellow cells " in some 

 Polyps and so important must be their influence 

 on their vital processes, that it may be confidently 

 asserted that the Polyps could not continue to 

 exist for long without them. In the genus Mille- 

 pora, for example, no single specimen and no single 

 fragment of a specimen that I have examined 

 was devoid of them ; and although the numbers 

 vary considerably the most superficial canals 



