112 COMPARATIVE ZOOLOGY. 
water. This may be called the “water- 
pipe system.” Besides this, there are nu- 
merous gill-like fringes, which probably 
aid in respiration. 
Tresh-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. 
All the rest of water-breathing animals 
have gills. The simplest form is seen in 
Marine Worms: delicate veins projecting 
through the skin make a series of arbores- 
cent tufts along the side of the body; as 
these float in the water, the blood is puri- 
Ma.17._Lob-worm fied. Bivalve Mollusks have four flat gills, 
(Arenicola piseato- consisting of delicate membranes filled with 
rum), a dorsibran- 
chiate, showing )lood-vessels 
the tufts of capil- 
laries, or external and covered 
Pests salnene. wiskinedliaey calm 
aes ae ty the Oyster, these 
ribbon-like folds are exposed 
to the water when the shell 
opens; but in the Clam, the 
mantle incloses them, forming 
a tube, called siphon, through 
which the water is driven by 
f 
— 
SM 
HE Oe a 
the ‘cilia.”. The aquatic ‘(Gas-! me. 70: Diaprammatic Section an 
re «we : Lamellibranch (Anodon): a, lobes of 
tel opods (Univ aly es) have mantle; 6, gills, showing transverse 
either tufts like the Worms partitions; ¢, ventricle of heart; d, 
: ’ bens ‘ : ? auricles ; e, pericardium; f, g, gland- 
or comb-like ciliated gills in ular sacs; h, venous sinus; hk, foot; 
y = A, branchial, or pallial, chamber; B, 
a cavity behind the head, to epibranchial chamber. 
