(i THE FKE8H- WATER I'ISIIKS OF EUROPE. 



cycloid type characterises Cypriiioid fishes, Sahnon, and the majority of fresh- 

 water forms. The scales exhibit their simplest development in the Eel, in 

 which they are remarkably thin, and consist of a sint^le layer of cells, which 

 preserve their forms distinct. The scales overlap each other in the majority of 

 fishes, so that only the posterior margins are exposed; but the size and number 

 vary in the different species, so that sometimes the scales are embedded in 

 skin without overlapping. They often vary in size on different parts of the 

 body. At times they are delicate and easily detached, while in other fishes 

 ^^^.-^^^^ their substance is strong-, and firmly adherent to the skin. 

 / %, I'sually the fins are free from scales, but in some types scales 

 }■ ,<^ extend on to the bases of the fins, and may more or less 

 '' •'. „,ii'^* completely cover the head. There is generally a row of per- 

 Fig. 1.— SCALE OF f orated scales extending along the middle of the side of the 

 fish, from the shoulder to the tail. This is known as the 

 lateral line. Its position is variable, and in many fishes runs parallel to the 

 dorsal contour, while in others it is parallel to the abdomen. As a rule, the 

 scales of the lateral line differ somewhat in form from the others, and may be 

 either smaller or larger. The lateral line is sometimes imperfectly developed, 

 and its development may be different in individuals of the same species. The 

 perforations on the scales are the outlet for the mucus canal, the secretion from 

 which lubricates the body of the fish (Fig. 2). This ^ 



canal is prolonged on to the head, and opens by pores a \ 

 on the lower jaw, and above and below the eye. 



^'7 

 A few fishes have no scales at all. The Sturgeon %''./ 



is covered with bony scutes, which, however, do 



. ' , . Fig. 2. SCALE Ol' LEUCISCIS. 



not overlap, and differ in size. 



The scales in fishes with a bony skeleton are arranged in transverse series. 

 The number of scales in the lateral line gives the number of these trans- 

 verse rows in most cases. The number of scales in a transverse series is 

 counted from the beginning of the dorsal lin downward and backward, so 

 as to follow the free borders of a line of scab's. The numbers above and 

 below the lateral line are written in the form of a fraction. Some fishes are 

 stated to shed their scales periodically. 



Sexsory Ougaxs. 

 The size of the eye is distinctive of many species. Its size is stated as a 

 third, fifth, seventh, or some other fraction of the length of the head. Its 

 ])osition in the head is defined by measuring the number of times its own 

 diameter is distant from the snout and from the other eye. There is some- 

 thing resembling an eyelid in the Salmon tribe, and in the Shad there are 



