HOW ANIMALS BREATHE. 113 
which the water is admitted by a siphon. The Cuttle-fish 
has similar flat gills covered by the mantle; but the water 
is drawn in by muscular contractions instead of by cilia. 
The end of the siphon through which it is ejected is 
called the funnel. The leaf-shaped gills of Lobsters and 
Crabs are also placed in tubular cavities, and a current is 
kept up by a little valve worked by the jaws. 
The perfection of apparatus for aquatic respiration is 
seen in Fishes. he gills are comb-like fringes supported 
on four or five bony or cartilaginous arches, and consist of 
myriads of microscopic capillaries, the object being to ex- 
pose the venous blood in a state of minute subdivision to 
streams of water. The gills are always covered; and the 
water taken in by the mouth passes between the gills, and 
escapes by a single opening on each side, in most Fishes, 
but by five slits in the Sharks. The act of “breathing 
water” resembles swallowing, only the water enters the 
gills instead of the gullet. 
(2) Air-breathers have trachee, or lungs. The former 
consist of two principal tubes, which pass from one end 
of the body to the other, opening on the surface by aper- 
tures, called spzracles, 
resembling a row of 
button-holes along the 
sides of the abdomen, 
and ramifying through 
the smallest and most 
delicate organs, so that 
the air may follow the 
blood wherever it cir- 
culates. To keep the 
pipes ever open, and 
at the same time leave 
them flexible, they are 
provided with an elastic spiral thread, like the rubber 
8 
Fie. 79.—Spiracle of an Insect, X 75. 
