358 - Multicellular Animals, Especially Man 



ing air or water, and the respiratory surface 

 is provided with an extensive network of 

 capillaries immediately beneath the surface. 

 At such a richly capillaried surface, equili- 

 brium occurs very quickly between the blood 

 and the environment; that is, oxygen quickly 

 enters and saturates the blood, whereas car- 

 bon dioxide leaves the blood, entering the 

 environment. 



In animals like the earthworm, the entire 

 body surface takes part in external respira- 

 tion, but this is not a very efficient arrange- 

 ment. To facilitate a rapid aeration of the 

 blood, the respiratory surface must be moist 

 and delicate, and this makes it susceptible to 

 external injury. The earthworm, for exam- 

 ple, cannot leave the protection of its bur- 



mouth 



ectoderm 

 endoderm 



gill slit 

 gill arch 



pharynx 



Fig. 19-1. Diagram of the gills of a fish (shark). 

 Arrows indicate the direction of flow of water. 



row, except for relatively short periods. Ex- 

 posure to sunlight shrivels and hardens the 

 delicate skin of the earthworm — and when 

 this occurs, respiration becomes inadequate. 



Gills. Most aquatic animals have devel- 

 oped gills (Fig. 19-1), which serve as special- 

 ized organs of external respiration. Typical 

 fish, for example, possess a hard scaly surface 

 covering most of the body. Such a surface 

 protects the individual from injurious con- 

 tacts, but it is not a suitable surface for 

 aeration. But the delicate gills of the fish 

 provide an excellent surface for the aeration 

 of the blood. The gills lie in a protected 

 position in the gill clefts, which lead from 

 the pharynx to the exterior of the body. The 

 fish takes in a continuous stream of water, 

 which passes through the mouth and out 

 through the gill clefts, bathing the soft fleshy 

 gills. Typically each gill is a lobulated mass, 

 which is well supplied with blood capillaries 

 (Fig. 19-2). This arrangement provides an 

 adequate surface across -which oxygen and 

 carbon dioxide are exchanged between the 

 blood passing through the gill and the water 

 passing over the gill. 



Lungs. Gills are not well suited to terres- 

 trial conditions because they are too exposed 

 and vulnerable to the drying effects of the 

 atmosphere. Instead of gills, many land ani- 

 mals have developed lungs. Essentially a lung 

 is a deeply inpocketed respiratory surface, 

 which is not exposed directly to the external 



Fig. 19-2. The larva of an 

 amphibian (Amblystoma 



punctatum) showing external 

 gills. Each gill is a fleshy out- 

 growth richly permeated by 

 capillaries, in which the blood 

 very quickly undergoes aera- 

 tion. (Courtesy of Roberts 

 Rugh, Columbia University.) 



