THE SEA SHORE 



43 



In all these cases we must remember that the conceal- 

 ment is not only from foes but also from the prey, i.e., 

 for attack as well as defence. For protection alone are the 

 limy, sandy or parchment-like tubes of the worms, while 

 the borers probably find the interior of stone or wood safer 

 than a more exposed habitat. 



The replacement of lost parts of the body is a very 

 familiar occurrence on the shore. Crustaceans have an 

 almost unlimited power of growing new limbs. It is quite 

 common for them to be faced with the alternative of 

 sacrificing one or more limbs or else losing their lives. 



4^iG. 5, — Diagram to illustrate breaking plane of Crustacean limb ; a, muscle 



responsible for breaking ; b, detached ring of third segment of limb ; /, breaking 



furrow (after Paul). 



They do not hesitate to do the former and the limb is cast 

 off at a special line of weakness called the " breaking plane," 

 the fracture being caused by the deliberate contraction of 

 the muscles at this point (Fig. 5). Owing to the depth of the 

 furrow, the wound is small and blood quickly coagulates 

 and closes the opening. So the animal remains, with a 

 stump in place of a limb, until the next time it moults, 

 when the rudiments of the new limb force their way out 

 quickly before the new shell has had time to harden. At 

 each successive moult the process is carried a little further 

 until the new limb is as large as those which were uninjured. 



