54 BRITISH FISU AND FISHERIES. 



forming Avhat is termed the lateral line. These 

 scales are pierced through near the centre by a 

 httle tube that allows the exudation of the glu- 

 tinous or mucous secretion, produced by glands 

 beneath, and which thence spreads over the 

 surface of the fish, defending it from the action 

 of the water ; this line is seen in the figure 

 at L. 



The gills of fishes are in general fringes on 

 gill-arches, as may be seen in the herring or 

 salmon, protected by gill-covers {operculum.) 



Each gill-cover, or operculum, is divided into 

 five parts: l,The pre-operculum; 2, The oper- 

 culum ; 3, The sub-operculum ; 4, The inter- 

 operculum ; 5, The branchiostegous * rays. 

 In some fishes, as the eel, the branchial aper- 

 ture is very small, and placed far behind the 

 * Gill-covering rays. 



