Introduction 



that performed by the skin). Within a short time after the ap- 

 pearance of the arms, the skin of the opercular membrane has 

 grown to that of the arm and a distinct seam is visible at the 

 line of union. (Figs. 289 and 229.) This seam is sometimes visible 

 in the second year after the metamorphosis. 



The metamorphosis may take place in a relatively short space 

 ot time, often being hastened by lack of food. Hunger causes 

 earlier degeneration and resorption of the edible portions of the 

 tail and opercular membrane, and hence hastens the metamor- 

 phosis. On the other hand, the change may be delayed by good 

 feeding and low temperature, and by any disturbance which 

 results in requiring the expenditure of energy on the part of the 

 tadpole. As the tail becomes smaller and no longer functional, 

 its colour darkens until it is nearly black. This is due to the 

 concentration of the pigment that was previously spread out, 

 giving colour to the tail and its fin. 



One of the most interesting structures of the tadpole is the 

 system of the lateral line organs. These are sense organs of the 

 skin that appear to the naked eye as dots of lighter or darker 

 colour, but are really tiny papillae at the ends of tubules of the 

 skin. They are arranged in curving rows on the top of the head 

 and around the eyes. They also extend backward to the tail or 

 beyond, usually in three irregular rows on each side. (Fig 4) 

 Their arrangement differs somewhat in the different genera and 

 species and they are more prominent in some than in others. In 

 a given individual they may be more conspicuous at some times 

 than at others, depending on the colour of the tadpole at the time 

 the observation is made. Each papilla is in direct connection 

 with a fibre from the lateral branch of the vagus nerve, and the 

 system of organs is supposed to provide a special sense necessary 

 in some way to aquatic life. Organs of the lateral line are uni- 

 formly possessed by Fishes; the organs of the lateral line possessed 

 by the Salientia are known in the tadpole stage only, and dis- 

 appear at the time of the metamorphosis. (See p. 14.) There 

 has been much difference of opinion as to the exact function of 

 these organs, but they are now regarded as refined organs of 

 touch, making the creature sensitive to vibrations of the mass 

 of water. They have developed from the skin, which is sensitive 

 to waves and currents of water. It is thought that the internal 

 ears have developed from organs of the lateral line, and the ears 



6 



