VERTEBRATA. 263 



is one of the Teleostei ; and the organs of the former 

 are developed in the head, while those of the latter are 

 near the tail. Other electric fishes have also been 

 described. The organs of the Torpedo consist of a 

 number of similar columns, transversely divided into 

 similar compartments ; each of these contains a layer 

 of very moist tissue, and a flat structure called the 

 " electrical plate," richly supplied with nerve-endings 

 somewhat comparable to the characteristic nerve-end- 

 ings which motor nerves form in muscular tissue. 

 The nerve-endings are distributed to the same side of 

 the plate throughout the organ, and the surface on 

 which they are found is electro-negative, while the 

 other is electro-positive; but how the energy of the 

 nerves gives rise to electric energy is not yet under- 

 stood. 



Amphibia. 



Links between Fishes and Amphibia. — We 



see, in the familiar form of the Tadpole, the most 

 striking instance of a link between fishes and am- 

 phibia. In the spring the student should get some 

 Tadpoles and study their successive stages. Some 

 adult amphibians retain certain fish-like characters. 

 The CcBcilim, for instance, snake-like animals found in 

 Brazil, have fish-like biconcave vertebrae with per- 

 sistent remains of the notochord. Those amphibians 

 which never lose their gills have also biconcave 

 vertebrae. The extinct animals called Lahyrinthodon 

 (from the complicated pattern of folds in their teeth) 



