124 THE BIOLOGY OF THE SEA-SHORE 



Autotomy and Regeneration in Echinoderms. — The 



faculty of self- amputation is, perhaps, still more widely dis- 

 tributed among Echinoderms than in the group we have 

 just been discussing, but it is not accompanied by the same 

 elaborate provision that is seen in the shore crab, for example. 



If the arm of the starfish Asterias is held in such a way 

 as to cause injury to the radial nerve, the animal will some- 

 times throw off the limb close to the base. The arm is 

 then gradually regenerated. Here, again, the tendency is 

 to regard the autotomy as adaptive. It is probable that the 

 sacrifice of an arm will sometimes enable the starfish to 

 escape, but whether it is legitimate to take this as furnishing 

 an explanation of the act is somewhat doubtful. The 

 relative frequency with which starfishes are found to have 

 autotomised limbs in natural circumstances certainly seems 

 to indicate that the autotomy is definitely of value to the 

 animal. Thus, King (1898) found that out of a total of 19 14 

 specimens of Asterias vulgaris there were 206, or nearly 

 II per cent., which had grown new arms, and with one 

 exception all of these arose close to the disc — evidence that 

 the arms had been autotomised and not merely torn off by 

 enemies. 



Not only is the place of an autotomised arm always taken 

 by a new one, but a single arm, provided it has a portion of 

 the disc still attached to it, will regenerate a new starfish. 

 Several workers have even found that in certain species of 

 starfish, including Linckia multifora and Asterina tenuispinay 

 a single arm, without any part of the disc attached to it 

 whatsoever, may produce a new animal. The so-called 

 " comet-forms," one of which is illustrated on Plate XI, 

 are forms which show a new set of arms regenerating from 

 a single arm in this way. In Asterias vulgaris, however, about 

 half of the disc requires to be left if the arm is to regenerate ; 

 but in exceptional cases one-fifth is apparently enough 

 (King, op. cit.). It has further been found by King that the 

 rate of regeneration is more rapid when the arm is auto- 

 tomised, in the normal manner, close to the disc, than when' 

 removed at a lower level. At first view this appears to be 



