EXOSKELETON 41 



It is therefore evident that the first bony hard substances 

 of the Vertebrata arise in connection with the external skin and 

 oral mucous membrane, and that the bony integumentary skeleton, 

 or exoskeleton, is therefore phylogenetically older than the bony 

 internal skeleton or endoskeleton. The latter owes its origin 

 to a gradual extension of the exoskeleton from the surface to 

 deeper parts, where it takes on relations to the cartilaginous endo- 

 skeleton. An independent ossification of the perichondrium, or 

 membrane which invests the cartilage, may also take place, so 

 that bone and cartilage now combine in the formation of the 

 skeletal framework and thus further complications arise. To 

 the original dermal ossification is added a formation of bone in 

 the perichondrium, and finally even a secondary endochondral 

 ossification may occur, replacing the cartilage : the former is most 

 marked in the Anamnia, the latter in the Amniota, and the result 

 of both is usually the subjection of the cartilaginous tissue in 

 the struggle of the tissues in the organism. 



In most Ganoids, thick plates, usually rhombic in form, are 

 present in the skin ; in the bony Ganoids (Polypterus, Lepidosteus) l 

 these cover the entire body, their margins being in apposition. 

 These ganoid scales correspond to the deeper part of the placoid 

 basal plates. Their surface is dense and smooth, owing to the 

 presence of a layer of gaiioin, of mesodermal origin, and formerly 

 erroneously described as enamel. The exoskeleton was largely 

 developed amongst fossil Ganoids. 



The scales of Teleosts are usually thin, and of the form known 

 as cycloid or ctenoid ; in the former the whole margin is smooth, 

 while in the latter the posterior margin is toothed and comb-like, 

 but various intermediate stages occur. The scales are arranged in 



FIG. 32. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE SKIN OF A 

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

 Zooloyy. ) 



I, derm ; o, epiderm ; s, scale. 



oblique rows and are situated directly beneath the epiderm, the 

 individual scales not touching one another. Secondarily, they 

 usually come to lie within definite pockets or sacs, and to overlap 

 one another like tiles on a roof (Figs. 13 and 32). The surface 

 of the scales may be sculptured. 



In the developing scale, a superficial dense portion, formed 



1 In Amia the scales have a "cycloid" form, and in the adult Polyodon 

 they are absent. 



