20 A PRIMER OF BIOLOGY 



essentially of protoplasm and a nucleus, is covered by 

 a cell-wall of relatively firm consistence, through 

 which no solid body can pass, and. further, it contains 

 one or more chloroplasts, absent, with very few 

 exceptions, from the cells of animals. Even where 

 they do occur there is good reason to believe that 

 they are in reality very simple plants allied to Pleuro- 

 coccus which have taken to living in the bodies of 

 these animals. We at once conclude that, as in 

 the case of Vaucheria, all the materials used by 

 Pleurococcus must be presented to it in the liquid 

 or gaseous state and must be absorbed by and passed 

 through the cell-wall. Moreover, these materials 

 are in the form of the relatively simple inorganic 

 bodies occurring in the situations where Pleurococcus 

 grows and must be constructed into organic com- 

 pounds before they can be assimilated by the proto- 

 plasm. This construction, as in the case of Vaucheria. 

 is associated with the presence of chloroplasts. 



Again, the existence of a more or less rigid 

 cell-wall renders movement in the case of Pleuro- 

 coccus impossible, although, in allied forms, the 

 plant is capable of movement at one stage in its life- 

 history through the agency of two motile threads 

 or cilia (Fig. 9B). Still, in general terms, we may 

 Nutrition note that a dependence on ready-made organic food 

 motion? " i g associated with locomotion, while the power of 

 constructing organic compounds out of inorganic 

 materials occurring in the soil and air accompanies 

 an absence of ability to move from place to place. This 

 important relationship, too often minimised or ignored, 

 will be discussed more fully later on (compare p. 68). 



Turning from unicellular forms, let us consider 

 the general structure of some organisms slightly 

 higher in the scale of life, e.g., a simple animal fre- 



