460 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



another, is shown in the figure (320), which is taken with 

 some simplification from Owen. 



Classification of Fishes by Scales.— Agassiz di- 

 vided fishes by the character of their scales into four 



Fig. 320.— Section through skin of a fish : d, dermis ; ml, Malpighian 

 layer; sc, scale. (From Owen.) 



orders— viz., the Ctenoids, the Cycloids, the Ganoids, and 

 the Placoids. Ctenoid scales are pectinate on their pos- 

 terior or exposed margin (Fig. 321, A). Cycloid scales 

 are smooth and rounded on this margin (Fig. 321, B). 

 Ganoid scales are bony, enameled, and usually rhom- 



D 



Fig. 321.— Fish scales, illustrating Agassiz's classification. 



boidal, close fitting, and not shingling (Fig. 321 Q 

 Placoid scales are very small, with outsticking sharp 

 points (Fig. 321, D). The first two are horny the last 



