STUDY OF FISHES 147 



sand-paper in the other direction. This is obviously due to 

 the fact that the fingers slip over the enamel tips in the first 

 case, and work against them in the second case. 



(2) In the sturgeon Acipenser there are five longitudinal 

 rows of very substantial bony plates, along with numerous small 

 spines. In the spines and on the surface of the large scales 

 the bone is usually replaced by a peculiar hard enamel-like sub- 

 stance called " ganoin." In the bony pike Lepidosteus of North 

 America the whole body is covered with what may be called 

 a chain-armour of four-sided bony scales articulating with one 

 another. Here again the bony scale is covered with a layer of 

 polished " ganoin." 



(3) In the ordinary food-fishes, such as cod and herring, 

 salmon and sole, the body is covered with thin transparent 

 scales which are often called " soft/' They overlap one another 

 like the slates on a roof ; they are embedded in, or sunk into 

 the dermis ; and their free portions are covered by the delicate 

 transparent epidermis which is over the whole body. Though 

 they are called soft and they are usually flexible they are 

 built up of bone-cells, and in many fishes they become hard 

 and spiny. Each really corresponds to the bony base of a 

 placoid scale. 



FlG. 52. The scale of an eel Anguilla vulgaris showing lines of growth by 

 which, in some fishes at least, the age of the fish can be determined. It may 

 be noted that inside the ear-cavities of bony fishes there are hard ear-stones or 

 otoliths, which increase in size as the fish grows older. Sections of these show 

 concentric lines from which the age may be read. In the common eel the 

 scales are so small that they are readily overlooked ; they occur in curious 

 groups embedded in the skin. In the conger eel there seem to be no scales at 

 all. 



Scales are absent in relatively few fishes, such as the electric 

 fishes (and most Silurids) ; they are represented by minute 

 papillae in the cling-fish (Lepadogaster) and Antennarius (e.g. 



