METHODS OF ATTACK AND DEFENCE 95 



able neither to fight nor to run away, are rendered incon- 

 spicuous by their close resemblance to their surroundings. 

 Such resemblance is usually regarded as having been 

 fostered by natural selection, and is therefore termed 

 " protective." It should be borne in mind, however, that 

 in many cases the term " protective " (like the term " adap- 

 tive ") is applied merely a priori^ and has no strict scientific 

 value unless the protective function has been demonstrated 

 experimentally. It frequently happens that an observer is 

 tempted to draw rash conclusions from the chance occur- 

 rence of an animal of a particular colour upon a seaweed of 

 the same hue and will allow cases of contrast to pass unheeded. 

 For instance, the Polychaet worm Eulalia viridis, when it 

 occurs on a green weed such as Ulva is quite inconspicuous, 

 but being an active species it occurs just as frequently on 

 red and brown seaweeds. 



We shall content ourselves with selecting for comment 

 a few of the many cases where a shore animal shows distinct 

 resemblance to its environment. 



Generally speaking, cases of protective resemblance arc 

 most frequent among Crustacea and Mollusca. The 

 commonest shore Ccelenterates, viz. anemones (particu- 

 larly Actinia equina), show a marked diversity of coloration 

 which is apparently quite incapable of explanation in terms 

 of the environment. Mortensen (19 14- 16), in his interesting 

 paper on protective adaptations and habits in marine animals, 

 states that worms, Echinoderms and Tunicates are all poor 

 in adaptive phenomena, whereas among Crustacea there occur 

 some of the finest cases of protective resemblance known. 

 The habit of masking in this group has already been dis- 

 cussed. Over and over again, when examining dredge 

 material on his Pacific expedition, Mortensen took for 

 pebbles what were really crabs. Crabs of the genus Parthe- 

 nope {Lamhrus) are wonderfully protected, some by long 

 claws resembling pieces of old coral branches while the body 

 looks like an ordinary pebble, others by the carapace being 

 expanded into flat crests covering the legs completely, the 

 animal from above looking like an old bivalve shell. On 



