in] OTHER DEFINITIONS 75 



gullet where it passes from the body of the Pilidium 

 into its own. The worm goes on its way rejoicing, 

 and grows up into an adult nemertine ; while the 

 Pilidium still swims about, though stomachless, for a 

 time, but perishes at the last. 



A perfect gradation in abruptness of metamor- 

 phosis can be traced up to this extreme condition. 

 Often, as in man, development proceeds gradually a 

 slow transition through continual change. In others, 

 as in the frog, there is one period when a sudden 

 alteration of habit and structure takes place; the 

 tadpole, we say, undergoes a metamorphosis and is 

 made a frog. But though there is radical rearrange- 

 ment, nothing is discarded. In the butterfly there is 

 a more violent metamorphosis, and also a part of the 

 earlier form, its outer skin, is discarded during the 

 change. Finally in the Pilidium not merely the skin 

 but nearly the whole of the larva is rejected at the 

 metamorphosis. 



From this, Prof. Huxley then says, it is but one 

 step for the larva to keep all its essential organs when 

 it parted company with the adult form ; the one would 

 be formed by the other after the fashion of a bud, and 

 from this on to the establishment of colonies like those 

 of the hydroid polyps would be but one step more. 

 Then we should have a perfect transition : the animal 

 as it develops is represented first by a succession of 

 forms, each one turning into the one that comes after, 



