410 THE SENSORIAL FUNCTIONS. 



the mode usual to the alligator, which is that of 

 snapping sideways at the objects he aims at de- 

 vouring. 



The organs of smell in Fishes are situated in 

 cavities, placed one on each side, in front of the 

 head : they are merely blind sacs, having no 

 communication with the mouth or throat, and 

 indeed no other outlet but the external openings, 

 which are generally two to each sac. The prin- 

 cipal entrance is furnished with a valve, formed 

 by a moveable membrane, appearing like a par- 

 tition dividing each nostril into two cavities, and 

 serving the purpose of preventing the introduc- 

 tion of any foreign body. The organ itself is 

 situated behind this valve, and consists either of 

 a membrane, curiously plaited into numerous 

 semicircular folds, or of tufted or arborescent 

 filaments. Fig. 388 shows this cavity (s), with 



its plaited membrane in the Perch; and Fig. 389, 

 in the Skate; the laminae in the former being 

 radiated, and in the latter, foliated, or parallel to 

 each other. On the surface of these organs, 

 whatever be their shape, the olfactory nerves 



