14 LIFE BY THE SEASHORE. 



difficult to distinguish from the coralline. There seems no 

 reason to helieve that such "protectively" coloured forms 

 are edible any more than the conspicuous forms, and they 

 do not attack active prey ; so that the use of the particular 

 coloration does not seem very clear. It is, however, certain 

 that a close resemblance between organism and surroundings 

 is a very common characteristic of shore animals, and doubtless 

 often conceals them from their enemies, and enables them 

 to steal unperceived upon their prey. In not a few cases 

 the coloration is variable, changing with the surroundings. 

 As groups in which this phenomenon may be looked for we 

 may mention Crustacea, such as crabs, shrimps, and their 

 allies; fishes, such as flounders, plaice, etc.; and even anemones, 

 such as the "cave-dweller" (see Fig. 25), Sagartia troglodytes, 

 whose colour varieties seem to show a relation to its sur- 

 roundings. 



In connection with this same subject we may notice the 

 habit of " masking " themselves which is displayed by many 

 Crustacea. Practically all the different kinds of spider- 

 crabs are found to have the back and legs covered by a 

 more or less thick coat of weed or zoophytes. These are 

 actually attached by the crabs themselves, as may be readily 

 seen in captivity, and are fastened on by very curious 

 hooked hairs with which the bodies of the crabs are covered. 

 The common Hyas araneus (see Fig. 55) of the East Coast 

 may be specially mentioned as a spider-crab which goes 

 about elaborately masked. Another form, Inachus dorset- 

 tensis, which lives in deeper water, shows a decided 

 preference for sponges, and is often found with back and 

 legs covered by masses of it. Curiously enough, the sponge 

 itself often has its interstices filled with the muddy burrows 

 of a little Crustacean (one of the Amphipods), which is at 

 times present in great numbers. 



These cases of "masking" pass by insensible gradations 

 into true symbiosis, where there is a constant association 

 between two animals, as in the cases noted above. 



There is one danger to which shore animals are subjected 

 which we have not as yet noticed, because although 

 doubtless they have acquired means of protection against 

 it, yet the adaptation is physiological, that is, a matter of 

 function, and cannot be studied as readily as a morpho- 



