GENERAL FACTS ABOUT SEA FISH 19 



bony fishes, and in some groups are provided with a " nicti- 

 tating membrane," which, as mentioned in the foregoing brief 

 contrast of the bass and tope, enables these sharks to close the 

 eye, when the light is too strong, by drawing up the lower 

 lid.* Rondelet appears to have been the first (in 1554) to 

 call attention to the fact of some sharks being able in this 

 fashion to shut their eyes, while others lack the power. The 

 organ known as " nictitating membrane " is most highly 

 developed in the genus Carcharias {e.g. the blue shark), and 

 least, where present at all, in Mustelus (e.g. the smooth 

 hound). In Galeus {e.g. the tope) the organ is moderately 

 well developed, but in Scyllium {e.g. the rowhound) it is no 

 more than a rudimentary fold of skin. Yet it acts even so, 

 for a smaller spotted dog-fish {Scyllium) in the Brighton 

 Aquarium was seen to shut its eye completely when on one 

 occasion a crab walked over its head. This nictitating 

 membrane is absent in the monk-fish {Rhina), spur-dog 

 {Acanthias), and rays {Raid). 



A curious bulging condition of the eyes of pout {Gadus) 

 brought up rapidly from deep water will be referred to in the 

 chapter on the cod family. 



Sometimes really deep-water fishes suffer far worse trouble 

 when abruptly brought into a region of diminished pressure ; 

 and the crew of the French vessel Talisman, which brought 

 back so many treasures of the deep to Paris some years ago, 

 hauled some fishes from the greatest depths to which their 

 baited hooks would reach, in which, as the fish came to the 

 surface, the dilated air-bladder was shot out of the mouth. 



The nostrils of fishes are only pits in the snout, and do not, 

 like our own, communicate with the throat. This does not, 



Tde however, impair the power of smell, for they must 



Nostrils, certainly be in touch with sensitive nerve-centres 



that convey their sensations to the brain. It has been argued 



* See Ridewood, Journ. of Anat. and Physiol., January, 1889, 

 pp. 228-242, and Harman, ibid., October, 1899, pp. 1-36. 



