HOW ANIMALS BREATHE. 



113 



there are numerous gill-like fringes, which 

 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 immedi- 

 ate contact with the soft skin, underneath 

 which lies a dense net -work of blood-ves- 

 sels. 



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. 64 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 



Fig. 11. — Lob-wonn 

 (Arenicolapiscato- 

 rum), a dorsibran- 

 chiate, showing 

 the tufts of capil- 

 laries, or external 

 gills. The large 

 head is without 

 eyes or jaws. 



Fig. 78. — Diagrammatic Section of a 

 Lamellibranch (Anodon): a, lobes of 

 mantle ; b, gills, showing transverse 

 partitions ; c, ventricle of heart ; d, 

 auricles ; e, pericardium ; /, g, kid- 

 neys; h, venous sinns; foot; A, 

 branchial, or pallia], chamber; B, 

 epibranchial chamber. 



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- 

 like ciliated gills in a cavity 

 behind the head, to which the 

 water is admitted by a siphon. 

 The Cuttle-fish has flat gills 

 covered by the mantle ; but the 

 8 



