SUMMARY OF THE FROG 119 



If the organs of different animals are morphologically equivalent, 

 that is, if they have developed in a similar manner in relation to the 

 other structures immediately surrounding them, they are called homo- 

 logous organs. 



There are three possibilities in comparing animals : 



(a) The organs may at the same time be homologous and 

 analogous. 



(b) They may be homologous but not analogous, as for exam- 

 ple the swim-bladder of fishes and lungs of mammals. 



(c) They may be analogous but not homologous, as for ex- 

 ample the gills of fishes and the lungs of mammals. 



The functions of organs are classified as follows : 



(a) Vegetative, found in plants. These have to do principally 

 with growth. 



(b) Animal, being those functions which are absent in plants 

 or but very slightly developed, while in the animal kingdom they are 

 considerably increased or are totally separate and distinct from any- 

 thing the vegetable world may possess.' 



The vegetative functions are equally complete in both man and the 

 lower animals although they may develop quite differently in the two 

 groups. 



Animal functions are those of motion and sensation. The work of 

 the various specialized sense organs, such as the eye and ear, come under 

 this grouping, while the work of those organs which pertain to nutri- 

 tion and reproduction, which both plants and animals possess equally 

 well, are vegetative. 



Living matter has been shown to have four distinguishing charac- 

 teristics: 



(a) Irritability, 



(b) Growth by intussusception, 



(c) Reproduction, 



(d) Nutrition. 



When nutrition is discussed biologically it must be thought of in 

 its widest sense as including not only the taking in of food and drink, 

 and the digestive process consisting largely of fermentation and the 

 absorption of such digested food, but also as including the taking in 

 of oxygen through the respiratory tract to cause heat and energy, and 

 the distribution through the circulatory system of the blood (the real 

 nourishment of the body) and finally, there must be included the ex- 

 cretory system which eliminates all that for which the 'body has no 

 further use. 



An organism was defined as any living thing capable of leading an 

 independent existence. 



If it is living matter it must have the four characteristics men- 

 tioned. 



