in] OTHER DEFINITIONS 79 



and develop gradually into the structures of the 

 perfect insect. 



In the Pilidium, it seems, the young worm finds 

 that less energy is wasted in feeding on its own 

 account than in attacking the larval tissues and 

 converting them into readily assimilable food-stuffs. 

 That part of the individual, therefore, which has been 

 so specialized for a free-swimming life as to defy 

 remodelling for worm-purposes, is discarded alto- 

 gether instead of being absorbed 1 . 



As with cells and organs, so with human beings : 

 it is rare that the skilled workman can change his 

 trade. When he is too specialized it may be easier 

 to give him notice and train a new apprentice than 

 to go through the pain and grief of the change from 

 fixed habits. 



The remains of the Pilidium then represent merely 

 a part of the Nemertine individual which is discarded 

 as being no longer useful : the history of the process 

 shows that it has nothing to do with ordinary asexual 

 reproduction such as the budding of 'polyps in a 



1 In one species of Pilidium (P. recurvatum Fewkes), however, the 

 young worm does actually absorb the remains of the larva. It is 

 interesting to note that a precisely similar series can be traced in the 

 metamorphosis of echinoderms. In the sea-cucumbers the process 

 is almost entirely one of remodelling, in sea-urchins and most 

 starfish the young imago is formed apparently as a "bud" and the 

 rest of the larva is absorbed later, while in some starfish (vide 

 J. Muller) the larva and the imago part company. 



