374 



ANIMAL DEFENCES 



do not forget, in so falling, to spin a thread of silk by which to climb 

 back to their home in case the manoeuvre is successful. Some 

 Caterpillars also, if alarmed when feeding on a branch, will sud- 

 denly let themselves drop and remain suspended by a thread of 



_ silk, and so also do some Slugs (fig. 

 510), though in their case the thread 

 is of different character. 



Many of the Crustacea are able 

 to retreat in an effective manner, and 

 this is obviously possible with power- 

 ful swimmers such as Lobsters and 

 Prawns, which, when pursued, make 

 at full speed for some sheltering hole 

 or crevice. The Common Shrimp 

 {Crangon vulgaris), and similar forms, 

 swim off quickly when alarmed, cloud- 

 ing the water as they do so by scuffling- 

 up the sand on which they live. Before 

 the water clears itself again they will 

 be found to have neatly buried them- 

 selves in the sand, though even then 

 they are not safe from fishes which 

 use the sense of smell to guide them 

 in the chase, as many do. Some of 

 the little crabs commonly seen on our 

 shores are pretty nimble, but their 

 pace is contemptible compared with 

 that of the Swift Sand- Crabs (Ocypo- 

 didcs) of African and American shores, which scuttle away at a 

 great rate when frightened, and are also able to bury themselves 

 in the sand with extreme rapidity. Crustacea are further distin- 

 guished by the readiness with which they part with their limbs, 

 and many of them have doubtless survived frequent hairbreadth 

 escapes by pursuing this policy, reminding one of the case of 

 Lizards already noted (see p. 371). Members lost in this or any 

 other way quickly sprout again. 



The power of regeneration alluded to above is present to a 

 much greater extent in segmented worms or Annelids. A marine, 

 freshwater, or terrestrial worm of this kind, if overtaken by an 

 enemy, is nevertheless not at the end of its resources, for even 



Fig. 510. — Slug suspended by a. thread ot 

 hardened slime 



