IIOW 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. 84 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- 
Fig. 78. —Diagrammatic Section of a like ciliated gills ill a Cavity 
Lamellibranch (Anodon): a, lobes of i i • J , winch the 
mantle; 6 , gills, showing transverse DemnQ tlie iieaa, lO WIHOJ 
partitions; c, ventricle of hoart; d, wa t e r is admitted by a sipllOIl. 
auricles; e, pericardium; g, kid- . r . 
neys; h, venous sinus ; k, foot; A, The Cuttle-fish lias flat glllS 
epibrShiaUhamber. chambe ‘ ’ B ’ covered by the mantle; but the 
8 
Fig. 77.—Lob-worm 
( Arenicolapiscato - 
rum), a dorsibran- 
chiate, showing 
the tufts of capil¬ 
laries, or external 
gills. The large 
head is without 
eyes or jaws. 
