HOW FISHES BREATHE. 21 



venous and impure blood of the body is forced. 

 The walls of these blood-vessels are of exceeding 

 thinness, so that the contained blood is brought 

 into very close contact with the water, which, 

 entering in at the mouth, is forced, as soon as 

 this is closed, through the slits in the wall 

 of the alimentary canal, and in thus escaping 

 bathes the vessels. The oxygen contained in the 

 air suspended in the water is seized upon by the 

 blood as the water flows past, and at the same 

 time the carbonic acid is given off and carried 

 away in the stream. And in this way, by the 

 passage of a stream of water over blood-vessels, 

 supported in the manner just described, is the 

 blood purified. 



The form of gill arrangement just sketched is 

 such as is found in, say, a perch or cod-fish. 

 J3ut in the sharks and dog-fish and rays, or 

 skates as they are often called, we find a yet 

 more primitive arrangement. Here each gill-slit 

 opens from the mouth into a kind of pouch ; and 

 the water which gains admission is forced out 

 through a slit in the outside of the animal. 

 Since there are a series of these slits, as in the 

 higher fish, so we get on the outside of the fish 

 a series of slits corresponding in number to those 

 on the inside — five to seven. These pouches are 

 formed by a double-walled partition or septum 

 extending outwards from every one of the solid 

 arches or half-hoops already described, to the 

 outer wall of the body. The rays or rods of the 

 arches run up between the double walls of each 

 septum. The opposite walls of each pouch sup- 

 port closely plaited folds of skin supplied with 



