58 



Basic Structure of Vertebrates 



times slender filaments (Fig. 57), richly filled with small blood- 

 vessels which lie so close to the surfaces of the lamellas or filaments 

 that the respiratory interchange of oxygen and carbon dioxide between 



BRAIN 



Fig. 58. The pharyngeal region of a 

 young Squalas (shark) embryo, showing 

 the visceral arches and clefts in frontal 

 section. (Courtesy, Neal and Rand: 

 "Chordate Anatomy," Philadelphia, The 

 Blakiston Company.) 



blood and external water readily occurs by diffusion through the ex- 

 ceedingly thin layers of tissue which separate them. 



Lungs are internal air-chambers which may be filled or emptied 

 via the nasal passages or mouth. Respiratory interchange of gases 

 occurs by diffusion between the air in the lung and blood contained in 



