114 
COMPARATIVE ZOOLOGY. 
water is drawn in by muscular contractions of the mantle 
instead of by cilia. The end of the siphon through which 
it is ejected is called the funnel. The gills of Lobsters and 
Crabs are placed in cavities covered by the sides of the shell 
(carapace); and the water is brought in from behind by the 
action of a scoop-shaped process attached to one of the 
jaws, which constantly bales the water out at the front. 
The perfection of apparatus for aquatic respiration is 
seen in Fishes. The gills are comb-like fringes supported 
on four or five bony or cartilaginous arches, and contain 
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. In 
bony fishes they are attached to the hinder side of bony 
arches, all covered by a flap of the skin, supported by 
bones (the gill-cover, or operculum ), and the water escapes 
from the opening left at its hinder edge. In Sharks, the 
gills are placed in pouches which open separately (Figs. 
164 and 287). The act of “breathing water” resembles 
swallowing, only the water passes over the surface of the 
gills instead of entering the gullet. 
( 2 ) Air-breathers have 
tracheae , 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 apertures, called spir¬ 
acles, resembling a row 
of button - holes along 
the sides of the thorax 
and abdomen, and rami¬ 
fying through the small¬ 
est and most delicate organs, so that the air may follow 
FiG. 79 — Spiracle of an Insect, x 75. 
