SKIN, SCALES, AND SPINES 91 



(crenulated), or the spines of a ctenoid scale may be soft and 

 scarcely noticeable, in which case the scale is spoken of as 

 ciliated. In some fishes the spines may extend on to the hinder 

 free portion of the scale, giving it a roughened appearance. 

 As a general rule, fishes with soft-rayed fins [e.g. Herring, 

 Salmon, Roach) have cycloid scales, whereas the scales are 

 ctenoid in the majority of Acanthopterygians {e.g. Perch, Bass) , 

 but exceptions to this rule are numerous. Both types of scale 

 may be developed on different parts of the body in the same fish. 

 Thus in many of the Sea Perches [Epinephelus) the scales above 

 the lateral line are mostly ctenoid and those below it cycloid, 

 and in the Dab {Limanda) the spiny ones occur on the upper or 

 coloured side, those on the blind or white side being quite smooth. 



The scales exhibit great diversity in shape in the different 

 species, ranging from the roughly circular to the long oval. 

 They also vary greatly in size. In the Tarpon [Megalops], for 

 example, each scale is more than two inches in diameter, and 

 these large structures are in some demand for ornamental work 

 (Fig. 39d) . Those of the Mahseer {Barbus) , the famous game-fish 

 of the rivers of India, are even larger, each being of the same size 

 as the human palm. At the other extreme we have the minute 

 cycloid scales of the Tunny {Thynnus) and Mackerel {Scomber), 

 and the microscopic scales of the Common Eel {Anguilla) . 



In fishes of the Herring family (Herring, Sprat, Pilchard, 

 Shad, etc.), the outer epidermal covering is very thin indeed, 

 and the scales, which are placed in shallow pockets, appear to 

 be lying on the surface of the body. Such scales are known as 

 deciduous, because of the ease with which they are rubbed off 

 when the fish is handled. In other fishes, of which the Plaice 

 {Pleuronectes) will serve as an example, they are more or less 

 deeply embedded in the skin. They are often also reduced in 

 size, and instead of overlapping, remain quite separate from 

 each other. The Common Eel {Anguilla) has a very slimy skin, 

 which is, to all appearances, quite naked, but if a piece be 

 examined under a microscope the presence of numerous 

 minute scales embedded therein is revealed. This is clearly 

 the result of degeneration as in the Plaice, and we are justified 

 in assuming that these scales are the remnants of once much 

 larger structures, which have gradually deteriorated as they 

 ceased to be of importance to the fishes. It may be noted here 

 that the ancient Hebrews, misled by the naked appearance of 

 the Eel's skin, included this species among the fishes forbidden 

 to them by Moses. His classification of the fishes into "all that 



