GREAT BRITAIN. 27 



compass as possible, while there are generally one or two 

 external openings situated on the snout. The capsule 

 which lines those depressions is formed of a fibrous mem- 

 brane, and which lies upon a cartilaginous or osseous base 

 termed the turbinal bones. 



The appearance of these openings or nostrils in bony 

 fishes is various ; generally two, and the anterior one often 

 tubular, the posterior more frequently oval, while a bridge 

 of integument occasionally provided with a valve, or a wide 

 interspace may exist between the two nostrils, which in- 

 ternally communicate with each other. In some eels, and 

 a few other bony fishes, one of the nasal orifices is on the 

 side of the upper lip. 



Among the plagiostomes the nasal depressions are very 

 large, and a membranous opercle is present by which they 

 may be closed ; the opening is on the under surface of the 

 face. The membrane lining them is very vascular, and 

 provided with crypts which secrete mucus. As the nasal 

 cartilage is provided with certain muscles, it has been 

 concluded that these fishes scent as well as smell, that is, 

 that they actively search for odoriferous impressions by 

 rapidly changing the current of water through the olfactory 

 sac. 



In the Cyclostomata there is a single nasal aperture 

 situated at the upper surface of the head ; in one form, the 

 myxine, or " hag," a communication with the palate exists 

 in the form of a naso-palatine canal, which opens backwards 

 on the palate, where it is provided with a valve. In the 

 stonesucker or lamprey (Petromyzon), the nasal duct 

 terminates in a blind pouch, and does not perforate the 

 palate. The organs of smell are supplied by the olfactory 

 nerve and a sensitive branch of the fifth pair. 



That fishes smell has long been known, and in olden 



