122 GENERAL PRINXIPLES OF ZOOLOGY 



SUinL\RY OF THE JIOST IMPORTAXT POINTS OF ORGANOLOGY. 



1. Organs are tissue eomplexcs, ditferenliated from the surrounding 

 structures by a definite form and adapted to the performance of a peculiar 

 function; consequently each organ can be classitied morphologically 

 (according to structure and relations) and physiologically (according to 

 function). 



2. Organs of different animals may be physiologically equivalent, 

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



3. Organs of different animals may be viorpliologically cqiiivalcitl, 

 homologous (developing in similar relations). 



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

 become evident, a. They may be at the same time homologous and 

 analogous, h. They mav be homologous, but not analogous (swim- 

 bladder of fishes, lungs of mammals), c. Thev may be analogous, but 

 not homologous (gills of fishes, lungs of mammals). 



5. Organs are divided into animal and vcgclalivc according to function. 



6. Animal functions are those which are only slightly developed in 

 plants; in the animal kingdom, on the contrary, they undergo an increase 

 and become characteristic. 



7. Vcgiialii'c functions are developed with equal completeness, 

 though in a different manner, in plants and animals. 



S. Animal organs include the organs of motion and sensation, such 

 as muscles, sense-organs, nervous system. 



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

 production. 



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

 taking in and digestion of food and drink, but also the taking in of o.wgeu 

 (respiration), the distribution of food to the parts of the body, and the 

 removal of matter which has liecome useless. 



11. With nutrition, therefore, are concerned not onlv the digestive 

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

 blood-vascular system, and the excretory organs (kidnevs). 



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



13. The male and female organs may occur in diiTerent individuals 

 {ditvrious), or both may be found in one and the same animal {hernia pliro- 

 dilir). 



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; wc then speak of a urogenital system. 



