132 GENERAL PRINCIPLES OF ZOOLOGY. 



characterized morphologically (according to its structure and its 

 relations) and physiologically (according to its function). 



2.- Organs of different animals may be physiologically equiva- 

 lent , analogous organs (i.e., with similar functions). 



3. Organs of different animals may be morphologically equiva- 

 lent, homologous (developing in similar relations). 



4. In the comparison of the organs of two animals three 

 possibilities become evident. 



a. They may be at the same time homologous and analogous. 



b. They may be 'homologous, but not analogous (swim-bladder 

 of fishes, lungs of mammals). 



c. They may be analogous, but not homologous (gills of fishes, 

 lungs of mammals). 



5. Organs are divided into animal and vegetative. 



6. Animal functions are those which are not completely foreign 

 to plants, but are only slightly developed in them; in the animal 

 kingdom, on the contrary, they undergo an increase and become 

 characteristic. 



7. Vegetative functions are developed with equal completeness, 

 though in a different manner, in plants and animals. 



8. To the animal organs belong the organs of motion and 

 sensation, such as the muscles, the sense-organs, the nervous 

 system. 



9. To the vegetative organs belong the organs of nutrition and 

 reproduction. 



10. Under nutrition, in the widest sense, are included not only 

 the taking in and digestion of food and drink, but also the taking 

 in of oxygen (respiration), the distribution of food to the parts of 

 the body, and the removal of matter which has become useless. 



11. With nutrition, therefore, are concerned not only the 

 digestive tract and its accessory glands, but also the organs of 

 respiration, the blood-vascular system, and the excretory organs 

 (kidneys). 



12. The male and female sexual organs serve for reproduction. 



13. The male and female organs may occur in different indi- 

 viduals (diwcious), or both may be found in one and the same 

 animal (hermaphroditic). 



14. The highest degree of hermaphroditism is attained when 

 one and the same gland (the hermaphroditic gland) gives rise to 

 both eggs and spermatozoa. 



15. Very often the sexual organs and the ducts from the 

 kidneys are closely united; we then speak of a urogenital system. 



