14 ZOOLOGY 



SECT. 



food being ingested than is required for simply maintaining the 

 size unaltered and soon a remarkable change takes place. The 

 processes become withdrawn, and a fissure appears dividing the 

 Amoeba into two parts (Fig. 2). This fissure grows inwards, and the 

 two parts become more and more completely separated from one 

 another till eventually the separation becomes complete, and we 

 have two distinct Amoeba? resulting from the division of the one. 

 While the protoplasm has been undergoing this division into two 

 halves the nucleus also divides, and each of the two new Amoebse 

 possesses a nucleus similar to the original one, and developed from 

 it by division. It is mainly by this simple process of division 

 into two, or Unary fission as it is called, that reproduction or 

 multiplication takes place in the Amoeba. 



In spite of the great simplicity of its structure, the Amoeba 

 thus carries on a number of different functions. The practically 

 structureless particle of protoplasm is able to act on matter 

 absorbed as food in such a way as to alter the chemical com- 

 position of the latter and to assimilate it : it is able to carry on 

 movements of locomotion as well as movements those involved 

 in the taking in of food particles which may be looked upon as 

 movements of prehension : it exhibits a certain degree of sensitive- 

 ness or irritability, as shown by the modifications of its movements 

 which result from contact with foreign bodies ; it is able to respire ; 

 it carries on processes of excretion ; and, finally, it is capable of 

 reproducing its kind. It is these functions that characterise living 

 beings as distinguished from non-living matter. The power of 

 locomotion, the capacity for assimilating organic substances, and 

 the absence of two special compounds chlorophyll and crlhdosc- 

 are specially characteristic of the animal as distinguished from the 

 plant. 



2. THE ANIMAL CELL. 



In all but the lowest animals the various functions just enume- 

 rated are carried on by means of a more or less complex machinery 

 of organs muscles, alimentary or enteric canal, glands, heart and 

 blood-vessels, gills or lungs, nervous system, organs of excretion, and 

 organs of reproduction. But in all animals, however complex, 

 the same substance, protoplasm, which in Amoeba constitutes 

 the bulk of the body, is the essential and active part. Wherever 

 in the body active functions are being discharged and active 

 changes are going on, there we find protoplasm present : where 

 there is no protoplasm there is no vital activity. In the 

 earliest stages of their existence all animals are formed entirely 

 of pi'otoplasm. Every animal consists at first of a single minute 

 particle of protoplasm, not widely different from an Amoeba. Soon 

 this particle divides into a number of parts which, instead of 



