LATERAL LINE OF FISHES 



So with the gills, however long they may last. It is now 

 believed that they are not, so to speak, retentions of the 

 fish gills, but gills peculiar to the larva of the amphibian. 

 To take a third feature in the anatomy of tadpoles : 

 they are most of them furnished with an elaborate 

 series of horny teeth within the mouth ; these " teeth " 

 are not later developed into the proper teeth of the frog, 

 nor do they persist when the tadpole leaves the water 

 and begins to croak. They are purely and simply larval 

 organs. In a few amphibians this nearly universal dis- 

 tinction between the two groups under consideration 

 is not to be met with. But if in a given tree-frog the 

 young frog is hatched at once from the egg as a frog, 

 without passing through the preliminary tadpole and 

 tree living stage, it does not follow that that tree-frog 

 must be removed from all other tree-frogs almost 

 exactly like it, and placed with the reptilia. In zoo- 

 logical classification we constantly come across examples 

 of animals most clearly affined to other animals and yet 

 lacking one or more of their important characteristics. 

 So much then for the difference between amphibia 

 and reptiles. There is no doubt that the amphibia 

 stand on a lower level than the reptiles, and that there 

 are distinct points of likeness to the fishes, some of which 

 have already been touched upon. In addition to those, 

 is the rather important fact that among the amphibia, 

 at least in the larval stages, is a representative of the 

 so-called " lateral line " of fishes. In any fish it can 

 be easily noted that a marked line runs along the sides 

 of the body often dividing the darker colour of the back 

 from the more silvery hues of the belly. This line is 

 formed of a series of sense organs of the nature of touch. 

 In spite of these various points of likeness between the 

 fishes and amphibians, there are profound differences. 

 The most profound is of course that all amphibians 

 agree with the animals lying higher than themselves 



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