XIV INTRODUCTION. 



Other important external characters are derived from the 

 operculum or gill-cover : a knowledge of its various parts, 

 and the names by which they are designated, may be learned 

 by a reference to the figure in vol. ii. page 3 ; where figure 

 1 marks the posterior edge of the preoperculum ; 2. the oper- 

 culum ; 8, the suboperculum ; 4, the interoperculum ; 5, the 

 branchiostegous rays. When a line or division occurs ante- 

 rior to the preoperculum, it marks the boundary of the cheek, 

 as in the heads of the fish represented in volume i. at pages 

 10 and 18, in which the different portions are not referred to 

 by figures. 



The use of the operculum is to close the aperture behind 

 the gills. The blood in fishes, while passing through the 

 gills or branchiae, receives the influence of oxygen from 

 water which enters by the mouth and goes out by this 

 aperture. In the fishes included in the first three orders, the 

 gills are so formed, and so freely suspended, that the water 

 bathes in its passage every part of their surface. 



In the Sturgeon, while swimming, respiration is carried on 

 in the same manner : but when the Sturgeon adheres to any 

 substance by the mouth which it has the power of doing by 

 extending its lips some other mode of respiration is re- 

 quired : and it is found that by the act of extending the 

 mouth the gill-covers are drawn up so as to leave a large 

 channel between them and the gills, through which the water 

 is brought into the mouth, and returned through the gills. 



In the Sharks and Rays, the temporal orifices probably 

 assist in the act of respiration by allowing entrance and egress 

 to water while the mouth is closed: they also enable the fish 

 to expel the water taken into the mouth with the prey pre- 

 vious to deglutition. 



In the Lampreys and Myxine, the branchial cells which 

 admit water are lined by the delicate membrane through 

 which the blood is aerated. In the Lampreys, the external 



