THE PHYLUM CHORDATA 



originated in ItcsIi water and remained there lor long ages, while the 

 sharks originated in the sea. Thus it seems most probable that both groups 

 were produced independently from placodcrm ancestors. The name of 

 the class is based upon its possession of a bony skeleton. While this is 

 a general character, yet it is variable, and som(^ "bony fishes" have largely 

 cartilaginous skeletons. Because of the extensive bony skeletons of ostraco- 

 derms and placoderms, that of tlu; Osteichthyes is now regarded as a 

 primitive character, while the cartilaginous skek;ton of the sharks and the 

 partially cartilaginous skeletons of some "bony fishes" are regarded as 

 l)ased upon secondary retention of an essentially embryonic character. 



Even at their first appearance in the fossil record, tlu; bony fisluss were 

 divided into two subclasses. One of these, the Choanichthyes (meaning 

 nose breathers), is of especial interest because it appears to have given 

 rise to the land vertebrates, while the other, the Actinopterygii, is of in- 

 terest because it includes the most successful of fishes. The Actinopterygii 

 take their name from the structur(; of their fins, which consist of a web 

 supported by more or less parallel skeletal rays. The basal bones of these 

 parallel fin rays are imbedded in the body wall, and the fin itself includes 

 little or no musculature. This is in contrast to the Choanichthyes, in which 

 the basal bones extend out longitudinally into the fin, with the fin rays 

 arranged radially around it. The fin musculature forms a mass over the 

 basals, and hence the Choanichthyes are often calkxl the "lob(!-finned" 

 fishes. 



Throughout the Paleozoic, the only actinopterygians were members of 

 the superorder Chondrostei. In the Devonian, they were very much out- 

 numbered by the Choanichthyes, but they expanded rapidly in the Missis- 

 sippian, and soon they were the dominant fishes of the fresh-water lakes 

 and streams. The skeleton of these fishes was largely ]K)ny. The external 

 armor was reduced to a coat of ganoid scales, bony plates covered with 

 a shiny, enamel-like substance called ganoine. The tail was shark-like. In 

 common with the Choanichthyes, these fishes had lungs. Embryological 

 evidence based upon living relatives of both groups of fishes and upon 

 land vertebrates indicates that the lungs originated as a modification of 

 the sixth gill pouch. The pouch first fail(;d to break through to the out- 

 side, thus forming a moist internal chamber lined with a respiratory mem- 

 brane. Swallowed air could thus be utilized for respiration in this in- 

 complete gill pouch. Subsequently, these modified gills migrated back 

 into the body cavity, thus becoming lungs. The lungs of tetrapods follow 

 this course of development, and the known facts seem to indicate that a 

 phylogenetic interpretation of the embryological data is justified in this 

 case. 



The Chondrostei reigned as the dominant fresh-water fishes until well 

 into the Triassic, when they were superseded by another superorder, the 

 Holostei. The chondrosteans contracted almost to the point of extinction 

 during the Mesozoic, but a few genera have persisted to the present. They 

 are represented at present in the Nile Valley by Polijplerus and Cala- 

 moichthys, and in the United States by the sturgeons and the spoonbill of 

 the Mississippi Valley. The sturgeons are widespread in the northern hemi- 



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