HOW FISH ARE CLOTHED. 35 



in the sharks and rays, we meet with the earliest 

 and simplest form of skin-covering, in the shape 

 of small bony bodies provided with a projecting 

 spine on the outer surfaces. The projecting 

 spine is the first part to be developed, and arises 

 from the outermost layer of the skin ; the basal 

 bony portion is developed later by the deeper 

 layer of the skin in which it is embedded. This 

 basal portion serves for the support of the spine. 

 At first these separate "placoid" scales are dis- 

 tributed unevenly over the body. Later in the 

 higher forms they arrange themselves definitely 

 in oblique rows, closely packed. The culminating 

 point in the arrangement of this solid scale type 

 is met with in those fishes once known as the 

 " ganoid fishes," of which we took the gar-pike of 

 the American rivers as a type. Here the scales 

 from mutual pressure have assumed either a 

 lozenge or a rhomboid shape, and for further 

 completeness of connection have developed peg 

 and socket joints. In the higher fishes, such as 

 the roach, perch, cod, herring, and so on, the 

 development of enamel by the outer surface of 

 the skin is dispensed with, the bony portion 

 formed in the deepest layers of the skin is greatly 

 reduced in thickness and otherwise modified, 

 resulting in a thin flexible plate, deeply em- 

 bedded in the skin by its anterior end, and 

 projecting backwards and outwards to overlap 

 its fellows on either side and behind, so as to 

 form the characteristic tile-Kke arrangement 

 with which we started. 



As birds renew their feathers by "moulting," 

 so many fish e.g. : salmon renew their scales by 



