16 THE ANGLER-NATUEALIST. 



by instinct to feeding near the bottom of water that is 

 often turbid and deep^ must necessarily experience more or 

 less imperfect vision from the deficiency of light. 



Fishes are subject to comparatively few diseases^ proba- 

 bly owing to the even temperature of the medium in which 

 they exist. They are^ however^ very liable to external and 

 internal parasites^ which (with the exception of those pecu- 

 liar to the Salmon) generally attach themselves to sick or 

 wounded fish. 



Smell. 



The sense of smell is considered by most comparative 

 anatomists to be very perfect. The nerves of smell are large, 

 and the extent of the membranes over which they expand 

 so considerable, that in a shark 25 feet long it has been 

 calculated at from 12 to 13 square feet. The discrimination 

 shown by fish in the selection and rejection of their ordi- 

 nary food, and the preference stated to be exhibited for 

 certain scented oils, &e., have been adduced as arguments 

 in favour of their acuteness of smell ; and there is no 

 question that sharks will pick out and follow a vessel 

 having slaves on board — a circumstance difficult to be 

 accounted for, except on the supposition that they discover 

 the nature of their cargo by scent. 



The nostrils, which are simple cavities near the end of 

 the muzzle, divided by a membrane, are generally pierced 

 with two holes, but both openings lead to the same common 

 canal. 



