40 ZOOLOGY. 



in size as before until it again collapses. It is evident that 

 some liquid is collected in it and periodically expelled from 

 the protoplasm. Hence the contractile vacuole, as it is 

 called, must be an excretory organ. 



4. Plant and Animal. Amoeba and the desmids on 

 which it feeds show us, in little, the whole relation of the 

 animal and vegetable kingdoms. The desmid, too, consists 

 of protoplasm with a nucleus, but it differs from amoeba in 

 two essential respects. Firstly, it has around it a distinct 

 cell- wall, composed of cellulose, which gives it a definite and 

 permanent shape, reduces its power of locomotion almost 

 to nothing, and renders it incapable of ingesting solid 

 food. Secondly, it has in its protoplasm a special portion 

 which contains the compound called chlorophyll, which 

 gives to it, as to other plants, its green colour. The 

 first of these two features is the greatest distinction between 

 all animals and all plants. The second distinguishes most 

 plants from most animals. 



The pond-water in which our two organisms live contains 

 in solution small quantities of salts ; such as various nitrates, 

 sulphates, phosphates and chlorides, and gases like carbon 

 dioxide and oxygen. This water diffuses into the plant- 

 protoplasm, and there the energy of sunlight, caught by 

 means of the chlorophyll, builds up from these simple com- 

 pounds complex molecules of carbo-hydrates and proteids, 

 oxygen being set free in the process. These are finally 

 elaborated into fresh protoplasm. Thus the plant-cell grows 

 and multiplies. These complex products are utilized by 

 the amoeba as food : they are no doubt first broken down 

 into somewhat simpler compounds in the process of digestion, 

 but these are then built up into the highly complex proto- 

 plasm of the animal. At the same time every bit of work 

 which the animal does and the formation of a pseudo- 

 podium is work involves the decomposition of some 

 of the protoplasm into simpler compounds containing 

 more oxygen. Thus the protoplasm of an amoeba is con- 

 stantly being increased by the anabolic process of food- 

 assimilation, and decreased by the katabolic processes 

 involved in all the work it performs. If the balance is on 



