iv SHIFTS FOR A LIVING 65 



for existence and much opportunity for device. Among 

 shore-crabs, especially, we find cases where the body is 

 covered by adventitious disguises, so that the real nature 

 of the creature is masked. Elsewhere, however, the 

 same may be seen ; the cases of the caddis-worms made 

 of sand particles, small stones, minute shells, or pieces 

 of bark serve at once for protection and concealment ; 

 the cocoons of various caterpillars are often masked by 

 extrinsic fragments. The nests of birds are often well 

 disguised with moss and lichen. 



But among marine animals masking is more frequent. 

 ' Certain sea-urchins," Prof. Poulton says, " cover them- 

 selves so completely with pebbles, bits of rock and shell, 

 that one can see nothing but a little heap of stones ; and 

 many marine molluscs have the same habits, accumulat- 

 ing sand upon the surface of the shell, or allowing a 

 dense growth of Alga? to cover them." 



This masking is in many cases quite involuntary. 

 Thus the freshwater snails (Limncea) may be so thickly 

 covered with Algse that they can hardly move, and some 

 marine forms are unable to favour or prevent the growth 

 of other organisms upon their shells. But how far this 

 is from being the whole story is well known to all who 

 are acquainted with our shore crabs. For though they 

 also may be involuntarily masked, there is ample evid- 

 ence that they sometimes disguise themselves. 



The hermit-crabs are to some extent masked within 

 their stolen shells, especially if these be covered by the 

 Hydroid Hydr actinia or other organisms. Various other 

 crabs (Stenorhynchns, Inachus, Maia, Dromia, Pisa) are 

 masked by the seaweeds, sponges, and zoophytes which 

 cover their carapace. Moreover, the interest of this mask- 

 ing is increased by the fact observed by Mr. Bateson at 

 Plvmouth that the crabs sometimes fix the seaweeds for 



tt 



themselves. Mr. Bateson describes how the crab seizes 

 a piece of weed, tears off a piece, chews the end in his 

 mouth, and then rubs it firmly on his head and legs until 

 it is caught by the curved hairs and fixed. " The whole 

 proceeding is most human and purposeful. Many sub- 

 stances, such as hydroids, sponges, Polyzoa, and weeds of 



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