HOW ANIMALS BREATHE. 



113 



there are sometimes numerous gill -like 



fringes, which cover the surface of the body 



and probably aid in respiration (Fig. 39). 

 Fresh-water Worms, like the Leech and 



Earth-worm, 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 con- 



tact 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 tufts 



along the side of the body; as these float 



in the water, the blood is purified.' 4 Bi- 



valve Mollusks have four flat gills, consist- 



ing of delicate membranes filled with blood- 



vessels and covered with cilia. In the Oys- FIG 77 ;_ Lo b- W orm 



ter, these ribbon-like folds are exposed to 



the water when 

 the shell opens; 

 but in the Clam, 

 the mantle en- 

 closes them, forming a tube, 

 called siphon, through which 

 the water is driven by the 

 cilia. The aquatic Gastero- 

 pods (Univalves) have either 

 tufts, like the Worms, or comb- 

 section of a like ciliated gills in a cavity 

 behind the head, to which the 



water is adiniued b y a si P hon - 



The Cuttle-fish has flat orills 

 i i_ i 



covered by the mantle ; but the 



8 



(Arenicolapiscato- 

 rwm), a dorsibran- 

 chiate, showing 

 the tufts of capil- 

 laries, or external 

 gills. The large 

 head is without 

 eyes or jaws. 



neys; h, venous sinus; k, foot ; A, 

 branchial, or pallia], chamber; B, 



i chamber. 



