ANIMAL PEDIGREES 233 



carapace, very similar to that of a Cypris. It 

 possesses a pair of well-developed antennules in 

 front and six pairs of swimming-legs behind. The 

 Nauplius eye is still present, a little way behind the 

 basal joints of the antennules. 



Now comes the great change. The pupa meet- 

 ing with a crab fastens itself to the under surface 

 of the crab's tail by its antennules, and then goes 

 to the bad with startling rapidity. Within three 

 hours of the time of fixing itself to the crab, the 

 six pairs of swimming legs, with the muscles 

 moving them, and the whole posterior part of 

 the body disintegrate and are cast off. The 

 antennules become modified into a tube, piercing 

 the skin of the crab ; the head of the Sacculina 

 remains as a bottle-shaped mass in connection with 

 the modified antennules, but the bivalved carapace, 

 with all the other organs including the eye, are 

 cast off and lost. The Sacculina now passes for a 

 time completely into the interior of the crab : later 

 on, after increasing in size, it comes once more to 

 the surface and becomes the bag-like mass which 

 we have found to be the adult condition. 



This is a typical instance of degeneration or 

 retrograde development, the animal being more 

 highly organised, and standing at a higher mor- 

 phological level in its early stages than when adult. 

 Yet inasmuch as the organs that are lost, such as 

 the limbs and eye, would be of no use to it in its 

 changed conditions of life, there is nothing in the 

 whole history that is in any way inconsistent with 

 natural selection. This principle of degeneration, 



