RESPIRATORY ORGANS. 



69 



Ct 



oxygen dissolved in water which is available. Hence this form of 



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 aiitler- 

 shaped or dendritically branched 

 processes (fig. 63 a, b), or of 



FIG. 63cr. Head and anterior body segments 

 of a Eunice, viewed from the dorsal sur- 

 face. T, tentacles. Ct, tentacular cirrus. 

 C, parapodial cirrus. Br, parapodial gill. 



lancet-shaped closely-packed leaves with a large 

 surface extension (fig. 64). 



Br 



FIG. 64. Transverse 

 section through the 

 gill of a Teleostean 

 fish, b, branchial leaf- 

 let with capillaries ; c, 

 branchial artery con- 

 taining venous blood ; 

 d, branchial vein con- 

 taining arterial blood. 

 a, branchial bar. 



FIG. 63i. Transverse section through the body of Eu- 

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

 bundle of seta? ; D, alimentary canal ; N, nervous 

 system 



The organs of aerial respiration, on thecontrary, 

 are internal. They present likewise the condi- 

 tion favourable for an exchange of gases between 

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

 surface. They have the form, either of lungs or 

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

 Vertebrates) they consist of spacious sacs with alveolar or spongy 



