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. 64 Bi¬ 
valve Mollusks have four flat gills, consist¬ 
ing of delicate membranes filled with blood¬ 
vessels and covered with cilia. In the Oys- PlG 77 ®_ Lob . worra 
ter, these ribbon-like folds are exposed to (Armicoiapncato- 
1 rum), a dorsibran- 
e c , the water when chiate, showing 
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 sinus; k, foot; A, 
branchial, or pallial, chamber; B, 
epibrauchial chamber. 
. in the tufts of capib 
the Shell opens; laries, or external 
but in the Clam, gjjj i8 Th £j t '3 
the mantle en- eyes or jaws, 
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 
