EESPIRATOHY ORGANS. 



09 



oxygen dissolved in water which is available. 



respiration is found in animals 



low in the scale of life in which 



the metabolic processes are less 



energetic (worms, molluscs, and 



fishes). 



Organs of aquatic respiration, 

 or gills, have the form of external 

 appendages possessing as large a 

 surface extension as possible. 

 They consist of simple or antler- 

 shaped or dendritically bi-anched 



Hence this form of 



Ct 



processes 



(fig- 



63 a, b), or of 



Fro. 63a . Head and anterior body segments 

 of a Eunice, viewed from the dorsal sur- 

 f-ace. T, tentacles. Ct, tentacular cirrus. 

 C, parapodial cirrus. Br, parapodial gill. 



lancet-shaped closely-packed leaves with a large 

 surface extension (fig. 64). 



FIG. 61. Transveroe 

 section througn the 

 gill of a Teloostean 

 fish, b, branchial leaf- 

 let with capillaries ; c, 

 1 ranchial artery con- 

 taining venoua blood ; 

 d, branchial vein con- 

 taining arterial blood. 

 a. branchial bar. 



FIG. G3i. Transverse section through the body of Eu- 

 nice. Br, gill ; C, cirrus ; P, parapodium with a 

 bundle of seta3 ; D, alimentary caiial ; A', nervous 

 system 



The organs of aerial respiration, on the contrary, 

 are internal. They present likewise the condi- 

 tion favourable for an exchange of gases between 

 the air and the blood, viz., a large extent of 

 surface. They have the form either of lungs or 

 sir-bearing tubes. In the first case (Spiders, 

 Vertebrates) they consist of spacious sacs with alveolar or spongy 



