CHAPTER XXV 



THE RESPIRATORY SYSTEM 



All chordates have a closed vascular system and haemoglobin 

 as a convenient transporter of oxygen. Their respiratory 

 systems involve structures in which blood-vessels are brought 

 into close contact with the surrounding medium (water or air) 

 with as little intervening tissue and as great an exposed surface 

 as possible. The former requirement is met by the very thin 

 nature of the epithelium covering the blood-vessels, and the 

 latter by reducing the size of the blood-vessels to capillaries, 

 which therefore have a large surface compared with their 

 volume. 



The respiration of embryos within their membranes is 

 effected by various means, such as the circulation of the yolk-sac 

 or of the allantois, as has been described in connexion with the 

 development of the frog, chick, and rabbit. 



After the embryonic stage has been passed, chordates 

 breathe either by gills, or by gills and lungs (sometimes 

 assisted by the skin), or by lungs alone. 



Gills are groups of capillaries in the walls of the gill-slits, 

 through which water passes out from the pharynx. In 

 Amphioxus the current of water is caused by the action of the 

 cilia on the under side of the oral^ hood and in the gill-slits 

 themselves. Fish breathe in the following manner : the gill- 

 slits are shut and the floor of the mouth is lowered, which 

 causes water to enter the mouth. The mouth is then closed, 

 its floor is raised, and the water escapes through the gill-slits. 

 When Cyclostomes are feeding, they are firmly attached to 

 their prey by their mouth and the sucker surrounding it. 

 They cannot therefore take in water through the mouth, and 



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