144 



ELEMENTS OF BIOLOGY 



SOME VERTEBRATE EXOSKELETONS 



DENTINE 



ENAMEL ORGAN 



DERMIS 



■ ENAMEL 



FIG. 91. PLACOID SCALE AND DIAGRAMMATIC SECTION 



BONY DERMAL SCALES 



HORNY EPIDERMAL SCUTES 



FIG. 92. CARAPACE lA) AND PLASTRON {B) OF TURTLE 



a 



RACHis 



QUILL 



VANE 



^gr AFTER SHAFT 



•INFERIOR UMBILICUS 



EPIDERMIS 

 DERMIS 



PAPILLA 



FIG. 93. CONTOUR FEATHER AND DIAGRAM OF FEATHER STRUCTURE 



Figs. 91, 92, and 93. — Some vertebrate exoskeletal structures. Placoid scales (Fig. 

 91) are characteristic of sharks and other elasmobranch fishes. A comparison of the 

 diagram of the placoid scale structure with that of a human tooth (Fig. 103) 

 shows them to be homologous. In Fig. 92 the horny epidermal scutes of the turtle 

 are outlined in heavy lines; the deeper dermal bony scales are lightly outlined. 

 (Part of Fig. 93 after Kiihn: Gnindriss der Allgemeinen Zoologie. Georg Theime, 

 Leipzig.) 



