170 REPRESENTATIVE SINGLE-CELLED ANIMALS 



which is sufficiently explained by the legend. It will be noted 

 that the individual rotates slowly on its long axis as it proceeds, 

 and that the response is a rather complicated one; although the 

 cell may seem to swing into line with the new source of illumina- 

 tion without much delay. A similar positive response to light 

 occurs in the free-swimming individual (Fig. 91 D). Just how the 

 euglena brings itself into the new position, whether by what might 

 be termed a " trial and error " method or by a more direct means, 

 as when a boat swings directly in response tc a " stimulation " of 

 the rudder, is a disputed question among investigators. This 

 can best be discussed for the protozoa as a whole in connection 

 with a species like Paramcecium, which is larger and more easily 

 observed (rf. p. 178). It is thus apparent that the euglenoid 

 cell responds to stimulation as does all protoplasm and hence 

 gives evidence of an irritability comparable with that shown by 

 the cells of higher organisms. 



Metabolic Processes. — The term holozoic nutrition is applied by 

 zoologists to the nutritive processes that occur typically in animals 

 and that have been described in a preceding chapter, as they 

 occur in the vertebrate, and again in the protozoan A?7iceba proteus. 

 Food, in the form of living prey or material derived from the bodies 

 of animals and plants, is ingested, and then digested to simpler 

 compounds, which are incorporated into the protoplasm by 

 assimilation. The green plants, on the other hand, exhibit such a 

 relationship to their surroundings that they are able to take in 

 simple compounds, such as oxygen, carbon dioxide, water, and the 

 mineral salts of the soil, and, by the process known as photosyn- 

 thesis because it is dependent upon sunlight, followed by other 

 synthetic processes, to build up the nutrients upon which their 

 existence depends. The substance known as chlorophyll, which 

 gives the green color to plants, is necessary for photosynthesis, 

 which is the initial step in this synthetic process. Nutrition of this 

 kind is referred to as holophytic in textbooks of zoology, although 

 this term is not in common use among botanists. 



What is known to occur in higher plants can be assumed to take 

 place in any cell that contains chlorophyll, and its occurrence can 

 in part be demonstrated by appropriate experiments upon unicel- 

 lular organisms. Euglena can be shown to liberate oxygen in the 

 presence of sunlight, as a result of photosynthetic processes carried 

 on by virtue of its chlorophyll. Starch, in the form of paramylum^ 



