FISHES 263 



at the mouth, and, passing back, streams outwards through the 

 gill -pouches thus passing over these folds, which collectively 

 present a large surface over which the oxygen dissolved in the 

 water can diffuse into the blood, while the waste carbon dioxide 

 of the blood can diffuse outward into the water and be carried 

 away to the exterior. 



In this connection great interest attaches to the spiracular 

 cleft, which looks very much like a narrow gill-pouch, .and on 

 close examination is seen to have a number of small folds on its 

 front wall. These are so gill-like that they have been collectively 

 called a false gill (pseudobranch), and when it is added that some 

 few fishes possess a properly-developed set of gill-folds in this 

 place, it will be clearly seen that the spiracular cleft is really a 

 gill-cleft which is losing its function. There is every reason 

 to believe that the cavity of the middle ear in higher Verte- 

 brates, together with the Eustachian tube, is equivalent to the 

 spiracular cleft. Here, then, is a structure which once had to 

 do with breathing and is now concerned with sound-conduction 

 another excellent example of change of function. 



Nervous System and Sense Organs (fig. 160). The brain is 

 pretty well developed, a peculiar feature being that the two 

 cerebral hemispheres are represented by an unpaired swelling, 

 while the olfactory lobes are placed on stalks. The cerebellum 

 is much larger than in Amphibia. 



The eyeball is flattened on the outside and its crystalline 

 lens is approximately spherical, as in aquatic animals generally. 

 Probably everyone has noticed, some time or other, the lens in 

 the eye of a cooked bony fish, such as salmon or herring, 

 looking, as it does, like a sugar-coated pill. Needless to say, 

 the opacity is the result of cooking. Here, too, the lens is 

 spheroidal. The organs of hearing consist simply of the internal 

 ear or membranous labyrinth, contained in a gristly capsule on 

 either side of the back end of the brain-case. In shape the 

 labyrinth is somewhat simpler than in the Amphibia. The skin 

 contains a large number of sense-organs, some of which are 

 sunk in a tube which runs along each side of the body and 

 opens to the exterior at intervals. An external streak, the 

 lateral line, marks the position of either tube, but this is much 

 better seen in a bony fish than in the Dog-Fish. There are 

 also peculiar jelly-tubes which open by regularly arranged pores 



