GREA T BRIT A IN. 23 



who keep fish in an aquarium are aware how, when the 

 water becomes foul or over a certain temperature, the con- 

 tained fishes rise to the surface and gulp down air direct. 



The respiratory current enters the mouth and is normally 

 discharged through the gill-openings, or by orifices situated 

 on either side of the head. But in embryonic and very 

 young forms a different plan may be pursued. In sharks 

 and their allies, wherein the gills are inside and attached to 

 the skin, a difficulty might arise sufficient oxygenation 

 might not be possible. To obviate this, there are tem- 

 porary external gills which are constantly bathed in the 

 surrounding fluid. To a certain extent the same plan is 

 likewise adopted in the bony fishes, as in the very young 

 the gill-covers do not cover the gills, which are conse- 

 quently immersed in the surrounding water and are thus 

 essentially external. 



The wider the gill-opening in a fish, the sooner, as a 

 general rule, does it expire after removal from its native 

 element ; examples of which may be seen in the herring, 

 pilchard, shad and mackerel. On the other hand, those 

 with narrow gill-openings usually live some time after their 

 removal from the water, as seen in the common eel. For 

 as the delicate fringes of the gills dry, they adhere one to 

 another, and this prevents their acting ; consequently, blood 

 cannot be decarbonised. Similarly fish may be suffocated 

 in very muddy water, their gills becoming choked. 



Fishes possess two eyes, placed symmetrically one on 

 either side of the head, and which are undoubtedly large if 

 compared with what obtains in other vertebrates, or even 

 in comparison with the extent of surface in their own 

 bodies. Their size, however, is modified in accordance 

 with local surroundings and their habits. Some frequent 

 muddy waters, where vision would be of less service than 



