HOW ANIMALS BEEATHE. 



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. Tlic 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 liave 

 gills. The simplest form is seen in Marine 

 Worms: delicate veins projecting through 

 the skiu make a series of arborescent tufts 

 along the side of the body; as these float 

 in the water, the blood is purifled." Bi- 

 valve Mollusks have four flat gills, consist- 

 ing of delicate membranes filled with blood- 

 vessels and covered with cilia. In the Oys- 

 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- 



Fio. 73.— Diagrammatic Section of a like ciliated gills in a Cavity 



mantle; 6, gills, showiog transverse behind the head, to which the 



partitions; o, ventricle of heart; rf, ^^tcr is admitted by a siphon. 

 auricles ; e, pencardmm ; /, g, kid- •' ^ 



neys; A, venous sinus ; *, foot;A, The Cuttlc-fish haS flat gills 

 branchial, or pallia], chamber; B, i , i i i i 



epibraiichiai chamber. covercd by the mantle ; but the 



8 



Fig. 77,~Lob-worm 



(^ArenicolapiscatO' 

 rwm)^ a dorsibran- 

 chiate, showing 

 the tuft;8 of capil- 

 laries, or external 

 gills. The large 

 head is without 

 eyes or jaws. 



