RESPIRATORY ORGANS OF AQUATIC ANIMALS. 271 



the gills composed of a collection of little leaf-like folds, placed 

 on a stalk (6, 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, o, a wide slit, is for the entrance 



Fig. 154. GILLS OF POULP. 



of water ; whilst the other, t, 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 FISHES, 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 car- 

 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 



