102 LIFE IN THE SEA [CH. 



with plant-animals, compound forms brought together 

 by mutual necessity. But there are also the cases of 

 many planktonic unicellular organisms, such as the 

 peridinians, and some others which are more difficult 

 to understand ; for although these organisms possess 

 chlorophyll corpuscles in their cells so that their 

 mode of nutrition is truly holophytic, yet their general 

 characters may suggest those of the animal rather 

 than the plant. The difficulty, however, is not one 

 that need concern us, for it may be the case that many 

 groups of the protozoa have evolved from chlorophyll- 

 containing organisms which we need not regard as 

 either plants or animals; and that some of them 

 while developing along the animal line have still 

 retained their plant-like mode of nutrition. 



Parasitism, to which we have already referred, is 

 essentially a mode of nutrition in which the process 

 of digestion is either eliminated entirely, or is greatly 

 abbreviated. In this condition two animals or two 

 plants are associated together, or an animal may be 

 associated with a plant. The association is often said 

 to be an obligatory one in that one of the associates, 

 the parasite, must live within the body, or on the body 

 of another, the host; although in few cases is the 

 actual contingency of the associates strictly necessary 

 for the life of the parasite. The parasitic habit is 

 very common among marine animals and we are able 

 to distinguish between two main groups of animals 



