86 PRINCIPLES OF ANIMAL BIOLOGY 



with these organs are all concerned in some way with digestion. They 

 constitute the digestive system. The heart, arteries, veins, and capil- 

 laries propel or convey the blood and so make up the circulatory system. 

 In like manner the brain, spinal cord, ganglia, and nerves compose the 

 nervous system. The term system is sometimes applied to a group of 

 organs of a single kind, when these are the only organs concerned with 

 that function. Thus, as will be explained in a later chapter, the excretory 

 organs of some of the simple animals (the earthworm, for example) 

 are all alike, but there are many of them. There is no objection to 

 speaking of these organs collectively as a system; but in all the more 

 complex animals the systems are everywhere made up of unlike parts, 

 each contributing a different portion of the general process. 



References 



Dahlgren, U., and W. A. Kepner. A Textbook of Principles of Animal Histology. 



The Macmillan Company. (Chap. VI, epithelium; Chap. VII, supporting and 



connective tissue.) 

 KiNGSLEY, J. S. Textbook of Vertebrate Zoology, Part I. Henry Holt & Company, 



Inc. (Pp. 9-16 for tissues.) 

 Storer, T. I. General Zoology. McGraw-Hill Book Company, Inc. (Pp. 17, 51-58 



for tissues; Chap. 4 for organs and systems.) 



