GENERAL CHARACTERS. 



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brates. The ova are small, numerous, usually pigmented, and 

 ■with yolk towards one pole. They are almost always laid in 

 water ; the segmentation is holoblastic, but unequal. There is 

 usually a metamorphosis in development. 



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 : — Gills are always present, but in Amphibians they may be 

 restricted to the larval stages ; there is no amnion, and at most a 

 homologue of the allantois ; there are lateral sensor}' 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 re- 

 presented, but may not persist in the adult life. 



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 ob- 

 vious differences are the absence of fin-rays and the development of fingers 

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



