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 
lias 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¬ 
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.—Diagram mafic Section of a like ciliated gills ill a Cavity 
behind the head, to which the 
partitions; c, ventricle of heart; a, W ater is admitted by a siphon. 
auricles; e, pericardium; /, g, kid- J 1 
neys; h, venous sinus; A;, foot; A, The Cuttle-fish. has flat gills 
branchial, or pallial, chamber; B, i 1 1 
epibranchiai chamber. 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. 
