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PHYLUM CHORDATA : CLASS AMPHIBIA 



Huxley was the first to recognise the affinities between Fishes and 

 Amphibians, and to unite the two classes under the title Ichthyopsida. 



Of the characters common to the two classes, the following are 

 important : (lill-slits are functional in respiration, but in Amphibians 

 they may disappear after larval life, the Eustachian tube excepted ; 

 gills are always present, but they may be restricted to the larval stages 

 in Amphibians ; in fishes and larval Amphibians a single ventral aorta 

 leaves the heart ; there is no amnion, and at most a homologue of the 

 allantois (in Amphibians) ; there are only ten pairs of cranial nerves ; 

 there are lateral sensory structures, such as the " branchial sense 

 organs " and those of the " lateral line," but these may be diminished 

 in the adults ; unpaired fins are almost always represented, but may not 

 persist in the adult life ; there is a functional pronephros in early stages. 



From the higher Vertebrates or Amniota the Ichthyopsida are clearly 

 distinguished by the presence of gills (in youth at least) and by the 

 absence of amnion and functional allantois. For though the bladder of 

 Amphibians may be homologous with an allantoic outgrowth, it does 

 not function as such, i.e. it does not aid in the respiration or the 

 nutrition of the embryo. 



It is more difficult to distinguish between Fishes and Amphibians, 

 more especially if we include the Dipnoi in the former class. The most 

 obvious differences are the absence of fin-rays and the development of 

 fingers and toes. In the following table the two classes are contrasted: — 



