SOME OTHER TYPES OF INDIVIDUAL ORGANIZATION 



185 



highly branched and ramifies throughout the body so that all other tissues 

 are within absorbing distance of the digested food. 



2. The Enteron. In all groups more complex than the flatworms, the 

 coelenteron, as a digestive cavity, is supplanted by a structure known 



inner ear 



spinal cord 



oviduct in female kidney 



71 



ureter 



pharynx with gill slits 



heart 

 esophagus llver gall bladder | 



bile duct 



spleen \ in,es,ine 

 pancreas r coelom 



arch of vertebra 



spinal cord 



vertebral 

 centrum 



dorsal rib 



ventral 

 rib 



igut 



Fig. 13.8. A vertebrate in diagrammatic longitudinal and cross section, to illustrate a highly 

 specialized organ-system construction. The small inset shows the portion of the body repre- 

 sented in longitudinal and cross section. {Modified from Romer, The Vertebrate Body, by 

 permission W. B. Saunders Company.) 



as the enteron, a digestive tube open at both ends, i.e., having both a 

 mouth and an anus. 



Digestion in the Earthworm. The enteron of the earthworm may be 

 used as an example of a digestive tube that has been differentiated to 

 form a system of organs. It consists of a mouth, opening into a buccal 

 cavity, a pharynx with strong muscular walls, an esophagus, a crop in 

 which food accumulates, a gizzard with thick muscular walls and a hard 



