276 THE VITAL FUNCTIONS. 



Death is, in like manner, the consequence of a 

 ligature passed round the fish, and preventing the 

 expansion of the branchiae and the motion of the 

 opercida. 



In all osseous fishes the opening under the oper- 

 culum for the exit of the respired water, is a simple 

 fissure ; but in most of the cartilaginous tribes, 

 there is no operculum, and the water escapes 

 through a series of apertures in the side of the 

 throat. Sharks have five oblong orifices of this 

 description, as may be seen in Fig. 367.* 



As the Lamprey employs its mouth more con- 

 stantly than other fish in laying hold of its prey, 

 and adhering to other bodies, the organs of respira- 

 tion are so constructed as to be independent of the 

 mouth in receiving the water. There are seven 

 external openings on each side (Fig. 368), leading 

 into the same number of separate oval pouches, 

 situated horizontally, and the inner membrane of 

 which has the same structure as gills : these pouches 

 are seen on a larger scale than in the preceding 

 figure, in Fig. 369. There is also an equal number 

 of internal openings, seen in the lower part of this 

 last figure, leading into a tube, the lower end of 

 which is closed, and the upper terminates by a 

 fringed edge in the oesophagus. The water which 

 is received by the seven lateral openings, enters at 

 one side, and after it has acted upon the gills, passes 

 round the projecting membranes. The greater part 

 makes its exit by the same orifices ; but a portion 

 escapes into the middle tube, and thence passes, 



* They are also visible in Fig. 293, (page 149), which is that of 

 of the Squalls pristis, a species belonging to this tribe. 



