12 CLASS XIV. 



On each side of the body lies a row of apertures, sometimes 

 extremities of bony tubes or of small pipes perforating the scales, 

 and forms the so-called lateral line (linea lateralis); under this 

 lateral line a glandular tissue, at least when these parts are more 

 developed, has been observed. The slimy fluid, with which the 

 body of fishes is covered, is secreted here. The line lies, in some 

 cases, closer to the back, in others, more towards the abdominal side ; 

 it is in some fishes (as Chromis, Xirichthys) interrupted, i. e. behind 

 it ceases on the back, and suddenly arises lower down, pursuing a 

 new direction on the tail. 



The head of fishes may, like the body, be narrow and compressed, 

 or flat and broad, naked or scaly. In the frog-fish (Lopliius pisca- 

 torius) it is almost as large as the body, in most fishes much smaller. 

 The jaws are not always equally long. In some, as in Xiphias, 

 the upper jaw, in others (especially in the genus Hemiramphus) 

 the under jaw is extraordinarily elongated. 



The aperture of the mouth in the sharks, rays and sturgeons 

 is situated on the under side, in most fishes at the anterior ex- 

 tremity of, the head. In the genus Uranoscopus the mouth is placed 

 upwards. The lips and the jaws have often various appendages, 

 filiform feelers (tentacula, cirri), &c. 



The teeth are distinguished by their form and the different 

 parts on which they are affixed. Besides the jaws, they may occur 

 on the vomer, on the palate, the tongue, the branchial arches, and 

 the small bones of the gullet. 



The nasal apertures and the eyes are the only organs of sense 

 that can be distinguished externally. In the genus of the soles 

 (Pleuronectes) the two eyes are placed on the same side of the head. 

 In others they lie one on each side, sometimes turned more towards 

 the upper surface (as in Uranoscopus), rarely more downwards, as 

 in the sucking-fish (Echeneis). 



The gills have ordinarily on each side behind the head a single 

 opening, through which the water, on expiration, is expelled (aper- 

 tura branchiarum) , and which mostly is in form of a semilunar 

 fissure, convex backwards. They are protected by bony plates as 

 a cover (operculum), of which the posterior margin is free. Beneath 

 this is a membrane, which is folded and supported by bony rays 

 that can expand it (membrana branchiostega). We shall advert to 

 these parts more particularly in the sequel. 



