132 GENERAL PRINCIPLES OF ZOOLOGY. 



fharacterized morphologically (according to its structure and its 

 relations) and physiologically (according to its function). 



2. Organs of different animals may be plujsiologicalhj equiva- 

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



3. Organs of different animals may he moriihologkalhj equiva- 

 lent, iLOinologous (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. 



]j. They may be homologous, but not analogous (swim-bladder 

 of fishes, lungs of mammals). 



r. 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. Verjetative functions are developed with equal eompleteuess, 

 though in a different manner, in plants and animals. 



S. To the animal organs Itelong the organs of motion and 

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

 system. 



!i. To the vegetative orga.ns belong the organs of nntritirm and 

 reproduction. 



10. f nder /(?i'/r/7('o;(, in the widest sense, are iiicluded not only 

 the taking in and digesti(^n of food and drink, but als(_i the takhig 

 in of oxvgen (respiration), the distribtition of food to the parts of 

 the body, and the removal of matter which has l)ec<>me itseless. 



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

 digestive tract and its accessoiy glands, Init also the organs of 

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

 (kidneys). 



];,'. The male and female se.xual organs serve for re]irodurriou. 



i:S. The male and female organs mav occur in diff'erent indi- 

 vidtials (ili(eci()us), or both may be found in one and the same 

 animal {hermapliroditic). 



14. The highest degree of hermaphroditism is attained when 

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

 both eggs and spermatozoa.. 



1."). Very often the sexual organs and tlie ducts from the 

 kidneys are closely united; wo then speak of a tirogenital svstem. 



