108 AN INTRODUCTION TO ZOOLOGY 



free life, obtaining its food for itself, but still has not attained 

 the form of the adult, that is while it is still a tadpole, it is called a 

 larva. The early history of the frog and the metamorphosis of the 

 larva are of importance, since they provide us with an actual example 

 of an animal changing from a gill-breathing swimming form into a 

 land-dwelling quadruped. This is a change that must have occurred 

 in the past when the fish-like Craniates took the all-important step 

 of forsaking the water for the land, and it proved to be the way to 

 the evolution of the higher craniate forms. 



Animals and Plants. 



In the frog we have seen and studied in some detail a fairly 

 typical example of a highly organised animal, and the examination 

 of its fundamental activities gives a good general idea of the charac- 

 teristics of the vital phenomena of an animal. Indeed, more than 

 this, the underlying principles are not merely the property of animal 

 life, but they are exhibited by all living beings, animal and plant. 

 Let us consider briefly these points, which fall into four groups. 



Firstly, all the living parts of an organism are composed of a 

 very complex substance, Protoplasm, which is a complicated 

 mixture of compounds termed proteins, themselves complex, 

 together with carbohydrates, fats, inorganic salts and water. Even 

 the lifeless products present, such as the hair, scales or feathers of 

 animals, or the bark of trees, are the results of the activities of this 

 wonderful substance protoplasm. It is, in the higher animals and 

 plants alike, split up into units, the cells, consisting of a proto- 

 plasmic body controlled by a denser nucleus and modified in almost 

 every conceivable way to perform different functions. One of the 

 main characteristics of protoplasm is its inherent power of con- 

 tractility or motility, which enables it to alter its shape and move. 

 This is probably brought about by a molecular rearrangement 

 accompanied by certain chemical changes, and although it is more 

 obvious in animals is nevertheless to be found in plants, and is a 

 fundamental property of all living matter. It is to be noted that 

 the alteration is in shape and not in volume, for the protoplasm 

 increases in one direction and decreases in another. 



Secondly, all living beings exhibit metabolism, and all that that 

 implies. They have the power of taking in dead or lifeless matter 

 and building it up into protoplasm by means of a series of changes, 

 digestion, absorption and assimilation, all implied in the term 

 nutrition. The energy required for carrying out this work is obtained 

 from the breaking down of the protoplasm itself, a process that is 

 dependent upon respiration, the process by which oxygen is supplied 

 to and utilised by the tissues. These processes result in the formation 



