74 ANIMAL INDIVIDUALITY [CH. 



inspection turns out to be a young nemertine worm, 

 wriggling actively inside a hollow sac which intervenes 

 between it and the tissues of the Pilidium. This is 

 strange ; but stranger still, the young worm contains 

 within itself the stomach that was the Pilidium's, so 

 that when the Pilidium feeds, the food passes through 

 its mouth into the stomach which is now the worm's. 

 The origin of the worm is equally curious : at a 

 certain stage in the growth of the Pilidium, five little 

 pockets appear on its outer surface, arranged in a 

 ring a little above the brim of the hat. The pockets 

 deepen, and their outer openings get narrower and 

 narrower, at length becoming quite "sewn up," so that 

 there are now five closed bags under the skin. These 

 bags flatten and then extend round the stomach of 

 the Pilidium in every direction, laterally as well as 

 up and down ; they thus meet each other, and the 

 walls which are in contact then disappear, so that all 

 their separate cavities join up into one. There is 

 now beneath the skin an outer shell, then a cavity, 

 and then an inner shell which surrounds the Pilidium's 

 stomach. This inner of the two shells or sacs becomes 

 thickened, undergoes various transformations, and at 

 last gives rise to the body wall and many other organs 

 of the young worm, while the outer sac is merely a 

 temporary protective envelope. The worm at length 

 wriggles so violently as to break through this envelope 

 and the skin of the Pilidium, meanwhile tearing the 



