32 COMPARATIVE ANATOMY 



surface of the body. In this stage their arrangement resembles that 

 seen in Ganoids. Subsequently they usually come to lie within 

 definite pockets or sacs, and to overlap one another like tiles on a 

 roof (Fig. 20 A). The surface of the scales may be sculptured. 1 



FIG. 20A. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE SKIN OF A 

 TELEOSTEAN, TO SHOW THE RELATION OF THE BONY SCALES. (Fro .11 Boas' s 



Zoology. ) 



/, dermis ; s, scale ; o, epidermis. 



Amongst the Siluridse (Fig. 20, B), Plectognathi, and Lopho- 

 branchii, they may be of relatively large size and so arranged as to 

 form a strong bony cuirass. 



Scales are wanting in Cyclostomes, and may be reduced or absent in repre- 

 sentatives of the three larger Orders described above (viz., in Electric Fishes 

 Spatularia, and some Eels). 



In the Dipnoi the arrangement of the scales is similar to that 

 seen in the Teleostei. They consist of an external hard substance 



FIG. 20u. DERMAL ARMATURE OF Callichthys. 



B, barbules ; BrF, pectoral fin ; BF, pelvic fin ; RF, dorsal fin ; DS and VS, 

 dorsal and ventral bony shields; t, lateral line. 



arranged in a network and provided with numerous denticles, and 

 of an internal portion composed of firm connective-tissue and bone. 



1 Different forms of the rounded or polygonal scales in Teleostei are dis- 

 tinguished as cycloid and ctenoid. The former, which are the more primitive, 

 have a smooth margin, while in the latter the posterior margin is toothed and 

 comb-like. Various intermediate stages exist between the two forms. 



