26 MANUAL OF ANIMAL BIOLOGY 



5. Adaptation 



An Amoeba has the power of adapting itself to its environ- 

 ment in response to various kinds of stimuli which it receives. The 

 protoplasm of which it is composed may be aroused or irritated by 

 a number of external factors such as light, temperature, electricity, 

 chemical stimuli, and contact. Any part of the ectoplasm of 

 Amoeba is able to receive these stimuli and to respond to them : it 

 is all irritable material. In higher forms of animals a specialized 

 irritable tissue, the nerve tissue, is present, which is connected 

 with, and forms, the essential part of various types of sense organs 

 adapted for receiving different kinds of external stimuli. 



The capture of food and the rejection of unsuitable materials 

 by an Amoeba, which have been noted above, may be taken as 

 examples of the ability of the ectoplasm of this animal to receive 

 and be influenced by external stimuli. It is probable that food is 

 secured in two ways : first, by the stimulus received through a 

 chance contact with a food particle and, second, as a result of a 

 chemical stimulus received from a distance. There is some evi- 

 dence to show that the Amoeba and certain other species of Proto- 

 zoa have the ability to ' sense ' food at a distance and then move 

 more or less directly toward it. 



C. General Facts of Importance 



Classification. In beginning our study of representative 

 animal forms which are of interest and importance from the 

 general biological standpoint, it will be helpful to understand 

 certain features of the systematic arrangement, or classification, 

 by which all organisms are placed in groups in accordance with 

 what appears to be their true relationships as determined by all 

 the available data. For efficient and systematic work in any field 

 it is always necessary to arrange, or classify, the particular objects 

 under consideration, whether they be living organisms or books 

 in a library. The system of classification necessarily becomes 

 more detailed and complex as the number of objects differing from 

 each other increases. If there is only one point of divergence, 

 then obviously only two groups are necessary, for all the objects 

 could be classified in one or the other group. (W. pp. 46-47.) 



In considering the animal kingdom we find that it is commonly 

 divided into thirteen primary groups, each of which is known as 



