MULTICELLULAR ANIMALS 



73 



muscles increases the rapidity of ventilation of the lungs. The in- 

 creased rate of breathing continues after exercise, until the chemistry 

 of the blood returns to normal. Many other events may stimulate 

 the respiratory centre of the brain, for example, fright, injury, and 

 shock; it is also under voluntary control. 



Food and Oxygen Transport Systems. The distribution 

 of nutritive materials from the digestive system and of oxygen from 

 the respiratory system, and the collecting of carbon dioxide and of 

 nitrogenous and other wastes for expulsion are effected by orderly 

 currents of liquids within the tissues. The complex system of trans- 



DORSAL BLOOD VESSEL 



DIGESTIVE 

 TRACT 



SUBNEURAL BLOOD 

 VESSEL 



EXTRA-OESOPHAGEAL VENTRAL BLOOD VESSEL 

 BLOOD VESSEL 



Fig. 1 1 6. — Diajjram of the circulatory system of the anterior region of an earthworm. 

 (Partly after Stephenson: Oligochaeta. Oxford University Press.) 



portation that constitutes the human circulatory system appears to 

 be the summation of various processes which take place in more 

 simple forms. The direct respiration and excretion in Protozoa and 

 in the simple diploblastic animals renders a circulatory medium 

 unnecessary; the surrounding water serves that purpose. In triplo- 

 biastic animals with deep tissues, watery circulatory liquids serve. In 

 the flatworms, Platyhelminthes, and in the Nemathelminthes the 

 circulatory liquid consists of a watery coelomic fluid that does not 

 appear to be exceptionally highly developed for carrying oxygen. 

 In most Mollusca, for example, the common oyster, and in Arthro- 

 poda, the circulatory liquid is in part confined in tubes and contains 

 a green compound, h.emocyax, containing copper, which by reac- 



