AUTOTOMY AND REGENERATION 127 



Structural considerations obviously preclude the possi- 

 bility of autotomy playing any important part in the life of 

 the sea-urchin. The spines, however, are continually being 

 shed in the aquarium, and new ones regenerated in their 

 place. Similarly, sea-urchins which have been captured by 

 the trawl are also found to grow new suits of spines. 

 New pedicellariae are also constantly being regenerated to 

 replace those lost in combat and in other ways. Prouho 

 (1887) found that if a fully-grown spine of Dorocidaris 

 papillata is broken off near the base the remainder is auto- 

 tomised and a new spine regenerated in its stead. 



Sea-cucumbers are remarkable for showing a very 

 thorough-going form of autotomy which, since it involves 

 the internal organs, is known as self-evisceration. When 

 a sea-cucumber is strongly irritated by any of a large range 

 of stimuli, the body contracts so violently as to compress the 

 fluid within the body cavity and cause the gut to tear, and 

 to be thrust out of the cloacal opening along with other 

 viscera. Among the first of the viscera to be extruded 

 are the Cuvierian organs or modified lower branches of the 

 respiratory trees. These are extraordinarily elastic and 

 sticky, and form a tough mass of white threads which look 

 like cotton ; hence the popular name of " cotton-spinners " 

 given to this group. In some species of Holothurian the 

 tearing of the gut is aided by a modification of the muscula- 

 ture of the gut wall just behind the gullet. This extra- 

 ordinary habit of self-evisceration has been the subject of a 

 considerable amount of speculation. Attempts have been 

 made, in particular, to find a purpose for the extrusion of the 

 Cuvierian organs and of their curious property of swelling 

 up in water. A widely-held view is that they serve for 

 protection, animals such as crabs and fish becoming entangled 

 in their meshes. Certainly, as Ludwig remarks, they cannot 

 be used for attack, since evisceration has never yet been seen 

 to be produced spontaneously, but only as the result of some 

 perfectly definite external stimulus. 



The drastic nature of self-evisceration in this group is 

 balanced by the remarkable capacity these animals have of 



