RESPIRATORY ORGANS 



251 



certain vertebrates (some teleosts, dipnoi, cyclos tomes), but recent 

 studies have shown its presence in many of these. In the case of 

 some teleosts it occurs as a slender tube in the mesentery; in the dip- 

 noi it is outside of the muscles in the intestinal wall, while in the cy- 

 clostomes it is partly concealed at 

 the insertion of the spiral valve, 

 partly (myxinoids) in the liver. In 

 these forms, owing to the complete 

 disappearance of the duct it becomes 

 a gland of internal secretion. The 

 pancreas may be elongate, compact, 

 or sometimes extremely lobulated. 

 Usually (fig. 267) it lies in a loop of 

 the duodenum. From certain pe- 

 culiarities of structure the question 

 has arisen as to whether two distinct 

 structures are included in the pan- 

 creas. 



THE RESPIRATORY ORGANS 



The respiratory organs have for 

 their purpose the exchange of gases 

 between the blood and the surround- 

 ing medium — water or air — carbonic 

 dioxide being given off and oxygen 

 being absorbed by the circulating 

 fluid. In order that the exchange 

 be readily effected it is necessary 

 that the organs be richly vascular, 

 that the walls between the blood 

 and the surrounding medium be ex- 

 tremely thin so as to permit rapid 

 osmosis, and that the osmotic sur- 



FiG. 270. — Pharyngeal region of a 

 young Acanihias embryo, b, blood- 

 vessels; c, ccElomic cavities of gill arches; 

 g, developing gills; gc, gill clefts; h, 

 hypophysis; tn, mouth; n, notochord; 0, 

 . . .1 1 T-. 1 oculomotor nerve; oe, oesophagus; p, 



face be as great as possible. Further, peritoneal cavity; s, spiracular cleft; 



there must be an adequate mechan- ^-^^^' ^"' ^° ^^""^ ^^^^ ^^^'^•'^• 

 ism for passing the oxygen-containing medium over the respiratory 

 surfaces. 



In the vertebrates the organs of respiration are developed in more 

 or less intimate connexion with the cephalic portion of the digestive 

 tract, just behind the cavity of the mouth. This part of the alimen- 



