THE SEA SHORE 53 



snell by adding to the free edges, or the sea urchins, which 

 increase theirs by adding new plates in between those 

 alreadj' formed, the crustaceans are unable to add to the 

 shell and have to cast it and form a new one (Plate 16). 

 These moults take place at regular intervals, depending on 

 the particular beast and on the prevailing conditions, the 

 entire shell being cast — not only that which protects the 

 body but the covering of the limbs and even the lining of 

 the stomach and hinder part of the gut ! The animal 

 withdraws itself, often with obvious difficulty, from its old 

 shell and is then left soft and completely defenceless, 

 until the new one has formed and hardened ; this, however, 

 takes place very quickly, for the substances of which it is 

 formed have been stored up in the skin beforehand. The 

 new shell is always larger than the old one, the animal 

 seizing this opportunity to " grow " to the size it would 

 have attained if the hard shell had not prevented its 

 increase. 



This must end our brief survey of the animals and plants 

 of the sea shore. 



