60 



MORPHOLOGY OF THE VERTEBRATE BODY 



dages numerous, and the organization wholly different, save for 

 the segmentation, the elongation of the body, the bilateral sym- 

 metry, and other very general features that arthropods and verte- 

 brates possess in common. 



K b.'d. u.bl. 



Fig. 29. — Diagrammatic longitudinal section of a vertebrate, with female 



reproductive organs. 



b.d., bile duct; br, brain; cl, cloaca; cop, coelome; y.s., gill slits; g.d., genital duct; h, 

 heart; I, liver; Ig, lung; m-n, mesonephros; n.t., neural canal; no, notochord; p, pan- 

 creas; pc, pericardium; p-n, pronephros; r.g., reproductive gland; s, spleen; s.c, spinal 

 cord; u.bl., urinary bladder; u.d., urinary duct. (Redrawn from Wiedersheim, "Com- 

 parative Anatomy of Vertebrates," copyright, 1907, by the Macmillan Co., printed by per- 

 mission.) 



The Structural and Functional Systems of Vertebrate Animab 



Organs and Systems. — In the terminology of Anatomy, the 

 body of an animal is composed of parts known as " organs," which 

 are grouped into " systems " according to their functional relation- 

 ships. Thus, one speaks of the muscular system, composed of 

 muscles, each of which can be regarded as an organ, although one 

 more commonly thinks of " organs " as parts hke the eye, the 

 stomach, or the heart. Again, there is the nervous system, compris- 

 ing the brain, spinal cord, and nerves, with the associated sense- 

 organs; the digestive system, composed of the digestive tract and 

 its appended glands, the liver and the pancreas; the skeletal, cir- 

 culatory, respiratory, excretory, and reproductive systems, with their 

 respective organs. Other systems might be included, but the fore- 

 going are the more famihar ones. The principal features of these 

 various systems may now be considered insofar as knowledge of 

 their structure is necessary for the discussion of functions given in 

 subsequent chapters. Since we are considering, in the present 

 chapter, the organization of the body from the standpoint of func- 



