110 VERTEBRATE ZOOLOGY 



from this type of scale are developed the true teeth the latter being 

 merely enlarged and flattened placoid scales derived from the oral in- 

 tegument. Thus the placoid scale is the ancestor of all true verte- 

 brate teeth. 



The ganoid scale is an archaic type of integumentary unit, found 

 only in the so-called ganoid orders of Teleostomi and in a few prim- 

 itive teleosts. The ganoid scale is believed to be derived from the 

 placoid condition by the loss of the spike-like protuberance and by 

 the addition of a hard external coating of glistening substance called 

 ganoin which is secreted by the dermis and is not homologous with 

 enamel. Usually ganoid scales are rhombic in form and are laid 

 like tiles, but in some ganoid fishes the scales overlap like shingles. 

 Cycloid and ctenoid scales are of similar structure to ganoid scales 

 except that they have lost the ganoin covering and are thinner and 

 less protective. Cycloid scales have a smooth circular margin and 

 are characteristic of the older groups of fishes, while ctenoid scales 

 have comb-like edges and are present mainly in the higher orders of 

 teleosts. 



The coloration of fishes is due to the presence of dermal pigment- 

 cells or chromatophores, which carry variously colored pigments and 

 are under the control of the central nervous system. The more prim- 

 itive groups of fishes are colored in rather neutral fashion and the 

 more highly specialized types are highly colored. The colors of trop- 

 ical fishes, especially those of the coral reefs, run riot and rival those 

 of the birds in elaborateness and brilliancy. Most of these highly 

 colored fishes belong to the climax order of modern fishes, the Acan- 

 thopterygii, in which the presence of high coloration is taken as evi- 

 dence of the onset of racial senescence. The flounders are the " cha- 

 meleons" among fishes. They are perhaps among all animals the 

 most efficient in their ability to modify their color patterns in re- 

 sponse to varied backgrounds. 



CLASSIFICATION OF PISCES 



SUB-CLASS I. ELASMOBRANCHII. 



Order I. Pleuropterygii (Family. Cladoselachidae) extinct. 

 Order II. Ichthyotomi (Family. Pleuracanthidse) extinct. 

 Order III. Acanthodei (Family 1. Diplacanthidae) extinct. 



(Family 2. Acanthodidae) extinct. 



