RESPIRATORY ORGANS OF AQUATIC ANIMALS. 271 
the gills composed of a collection of little leaf-like folds, placed 
on a stalk (8, fig. 154); they are inclosed in a cavity which is 
covered-in by the mantle ; and the walls of this cavity have 
the power of alternately dilating and contracting, so as to 
draw-in and expel water. It communicates with the exterior 
by two orifices, one of which, 0, a wide slit, is for the entrance 
Fig. 154.—GiLLs oF PouLr, 
of water ; whilst the other, ¢, is tube-like, and serves not only 
to carry-off the water that has passed over the gills, but also 
to convey away the excrements, and the fluid ejected by the 
ink-bag. This is called the funnel. 
317. In Fisues, the gills are composed of fringes, which 
are disposed in rows on each side of the throat, and are 
covered by the skin. The cavity in which they lie has two 
sets of apertures ; one communicating with the throat, and 
the other opening on the outside. In the Fishes with a ear- 
tilaginous skeleton, we usually find as many of these external 
orifices as there are rows of gills; thus in the Lamprey there 
_ are seven, as shown in the succeeding figure (a). But in 
_ Fishes with a bony skeleton, there is usually but a single 
_ large orifice on either side ; and this is covered with a large 
_ valve-like flap, which is termed the operculum or gill-cover. 
_ A continual stream of water is made to pass over the gills by 
_ the action of the mouth, which takes-in a large quantity of 
