4 THE FRESH- WATEi; FISIIE8 UF EFKOrE. 



distinction between sncli varieties and species, or to make a definite demar- 

 cation and group the varieties togjether, by taking- gome character which they 

 have in common as the basis for a species. Thns there are many kinds of 

 Charr, but we mig-ht easily ^roup them tog'other, and, in the same way, the 

 g-radations between the Trout wliich do not migrate, would al^sorb a lar^e 

 number of types which have usually lieen re^jjarded as species. Fish characters 

 beiui^ so variable, and chang-in*^ with ag^e, it l)ecomes necessary for us briefly 

 to examine them, so that their nature and value may be observed in living 



fishes. 



Fish Form. 



There is less variety in the aspect of fresh-water fishes than in the larg-er 

 assemblage which inhabits the sea. There are none of the marvellous modifi- 

 cations of snout, such as distinguish the Sword-fish and the Saw-fish ; none 

 of the singular conditions of fin which are seen in crawling Gurnards, or the 

 fins of Flying Fishes. Nor are the fins ever modified into a sucker, as in 

 several marine types. There are no such singularly-expanded fresh-water 

 fishes in Europe as the Rays, no globular expansible spiny globe-fish like the 

 Diodons, or armoured box-fish like Ostracion, or Ribbon Fish, or Sea-horses, or 

 Sun-fish.* And even the most singular inhabitants of rivers are fishes which 

 come up from the sea, like Lampreys, Eels, Sturgeon, and Flounders. If we 

 exclude these genera, fresh-water fishes may be said all to possess the typical 

 fish form, which is compressed from side to side, widest behind the head, 

 tapering towards the tail, with the caudal fin evenly lobed, the other fins well 

 developed, and the body, in most cases, covered with scales. The parts of the 

 bod}' are described as the dorsal region or back, sides, and abdomen. The 

 proportions of the body furnish important characters in distinguishing genera 

 and species. These are stated in the relation of the length of the head to the 

 length of the fish, and in the proportions of depth and thickness of the body 

 to its length. The abdomen may be rounded or form a shai'p edge. The 

 relative positions of the fins also furnish distinctive characters. 



Tjie Fins. 

 Fresh-water fishes nearly all have two pairs of fins, which may be regarded 

 as corresponding to the limbs of terrestrial animals (Fig. -J). The pair at the 

 hinder margins of the head are termed pectoral fins. The pair beneath them, 

 and usually placed farther back, are the ventral tins. Besides these, a fish has 

 at least three other limbs, which are vertical. There are one or two dorsal tins 

 on the median line of the back, but when there are two, one is placed behind the 

 other. More or less opposite to the dorsal fin, but always behind the vent, is 



* Sop Article " Fishos," in " Ca^^sr^ll's Natuiiil IIist(n-y." 



