HOW ANIMALS BREATHE 



319 



mals breathe air directly ; the rest, with few exceptions, 

 receive it through the medium of water. In the former 

 case, the organ is internal ; in the latter, it is more or less 

 on the outside. But however varied the organs tubes, 

 gills, or lungs they are all constructed 

 on the same principle a thin membrane 

 separating the blood from the atmosphere. 



(i) Protozoa, Sponges, and Polyps have no 

 separate respiratory apparatus, but absorb 

 air, as well as food, from the currents of 

 water passing through them or bathing 

 the surface of their bodies. 



In the starfish, sea urchin, and the 

 like, we find the first distinct respiratory 

 organs, although none are exclusively 

 devoted to respiration. There are two 

 sets of canals one carrying the nutrient 

 fluid, and the other, radiating from a ring 

 around the mouth, distributing aerated 

 water, used for locomotion as well as 

 respiration. This may be called the 

 "water-pipe system." Besides this, there 

 are sometimes numerous gill-like fringes, 

 which cover the surface of the body and FIG. 2 74 . Lobworm 



v 1.1 ' i '4.' 



probably aid in respiration. 



Freshwater worms, like the leech and 

 earthworm, breathe by the skin. The 

 body is -always covered by a viscid fluid, 

 which has the property of absorbing air. 

 The air is therefore brought into immediate contact 

 with the soft skin, underneath which lies a dense net- 

 work of blood vessels. 



But most water-breathing animals have gills. The 

 simplest form is seen in marine worms : delicate veins 

 projecting through the skin make a series of arborescent 



(A renicola piscato- 

 rum) t a dorsibran- 

 chiate, showing the 

 tufts of capillaries, 

 or external gills. 

 The large head is 

 without eyes or 

 jaws. 



