ORGANIC STRUCTURE: MORPHOLOGICAL 45 



present food substances that can be directly absorbed through 

 the integument (being changed chemically in the processes of 

 absorption). 



Mouth 

 glcuioLs 



StoTTvcuch & Glands 



MouJtk 



Blood vessels cirounoi 

 '' cdime-ntccnzi canal 



I 





euh 



Arteriole 



CapilLoiri/ 



plejcus 



Air sac'' 



Tissu es—- 



"T' > nil I M M 'i '. 



around air tubes 



Water orodr 

 CapUJLarif plejcus 



Fig. II. 



I, The alimentary canal, with its glands in a vertebrate ; 2, a respiratory tentacle; 3, an 

 element of the gill of a fish ; 4, an air-sac in a mammalian lung ; 5, a tracheal tube in an 

 insect ; 6, the essential respiratory mechanism. 



\2c. Organs of Respiration. In all cases there is a thin, 

 semi-permeable, respiratory membrane. On one side of the 

 membrane is the respiratory medium, air or water containing 

 oxygen. On the other side is a plexus of blood-vessels. Oxygen 

 is absorbed from the medium through the membrane and CO 2, 

 etc., are extruded from the blood to the medium. This is the 

 essential respiratory mechanism in lungs, gills, respiratory plumes, 

 the tracheae of insects, etc. 



\2d. Organs of Circulation. The fluid (blood) that carries 

 nutritive materials to the organs of motion, etc., is conveyed 

 by blood-vessels, arteries, capillaries, veins and channels of 

 less definite formation. The fluid is propelled through the 

 channels (blood-vascular system) by the pumping mechanism, 

 or heart. 



There are innumerable variants of such a mechanism : for 

 instances, the heart may be double, triple or quadruple- 



