SOME OTHER TYPES OF INDIVIDUAL ORGANIZATION 



183 



groups working together as tissues, and tissues cooperating in the forma- 

 tion of organs — the organs themselves are, for the most part, grouped 

 into systems, each system being adapted for the performance of one or a 

 few functions. The human body is constructed on this plan, and in Chap. 

 II its hierarchy of cells, tissues, organs, and systems was described. 



supraphoryngeal ganglion 



subpharyngeal \ \ he 



ganglion pharynx \ 



ventral nerve cord 



longitudinal muscles dorsa | pore 



esophagus gizzard 



cuticle 



ypodermis 



circular muscles 

 — ^setae 

 ryphlosole 



intestine 



coelom 



ventral blood vessel 



ventral nerve cord 



subneural blood vessel 

 setae 



nephridium 



Fig. 13.7. An earthworm (Lumbricus) in diagrammatic longitudinal and cross section, to 

 illustrate a simple organ-system construction. The small inset shows the part of the body 

 longitudinally sectioned; the cross section is made farther back. 



Animals whose organization has attained this level share certain funda- 

 mental characteristics in spite of their many differences. In all of them 

 the bulk of the body is formed from the third cell layer (mesoderm), 

 which we first noted in the flatworms. They all have a body space sur- 

 rounding the digestive tract, so that the plan of the body is essentially 

 that of a tube within a tube. Further than this it becomes difficult to 

 generalize, because the structural patterns of the higher Metazoa have 

 diverged along a number of quite distinct lines, related to different modes 

 of development, habitat, and ways of life. All, however, have attained 



