44 



THE SEAS 



This habit of deliberately parting with limbs which are later 

 regenerated is called " autotomy," and is not confined to the 

 crustaceans, being widespread amongst the starfish and 

 their delicate allies, the brittlestars. Both of these part 

 with their arms very readily and quickly grow new ones ; 

 they may lose all their arms and yet from the central disc 

 there will grow out a complete new set (Fig. 6). 



The related sea gherkins have the more unique power of 



casting up their viscera 

 when disturbed or attacked , 

 then proceeding at their 

 leisure to grow new ones. 

 It may well be that, under 

 normal conditions, the 

 attacker is satisfied with 

 the meal of soft entrails 

 thus provided, and will not 

 trouble their owner further. 

 The worms have excep- 

 tional powers of regenera- 

 tion and when cut in two 

 accidentally — or by design 



Fig. 6.— Starfish regenerating arms. in the laboratory — will 



Two stages in regeneration of new arms -, A 4-nilo 



from a single severed arm (adapted from g row new neaaS Or tails 



Flattely and Walton). with equal facility. 



Sponges have probably the most remarkable powers in 

 this direction, for a sponge can be broken into tiny 

 fragments which are then strained through fine meshed 

 silk, and yet the isolated pieces will come together and 

 unite to form new individuals ! 



We are leaving a consideration of parasitism and of the 

 intimate, mutually advantageous union of two animals or 

 an animal and a plant, known as symbiosis, to Chapter IX, 

 but we must say something here of that looser form of 



