METHODS OF ATTACK AND DEFENCE 91 



forms of the same species. For instance, mussel beds are 

 much subject to overcrowding, the older forms becoming 

 gradually suffocated beneath an ever-growing mass of 

 younger individuals. 



The mollusc Crepidula normally attaches itself to other 

 shells, either of the same or of unrelated species, and as 

 many as five individuals may occur one on top of the other 

 in a manner that has been likened to a sky-scraper. 



It would be unwise to conclude from the preceding 

 account that epizoic forms invariably constitute a menace 

 to their host, but it is difficult to determine at what point 

 the association ceases to be harmless. Undoubtedly, en- 

 crusting forms are often of considerable benefit in helping 

 to hide the underlying species or to render it less conspicuous. 

 Not only so, but cases occur in which active encouragement 

 is given to epizoic forms. In short, there are three possi- 

 bilities : an epizoic association may be trivial, indifferent 

 and may remain so, or it may be seized upon and utilised 

 or may sink into a drawback. 



TACTICS OF THE SHORE STRUGGLE 



Masking. — The long-legged spider-crabs {e.g. Inachus^ 

 Hyas) have evolved the habit of planting out upon their 

 hairy carapaces all kinds of seaweeds, hydroids and the 

 like, the result of which is to render them almost invisible 

 in their normal environment. Specimens with their bodies 

 covered with algae, when placed in a tank among sponges 

 have even been observed to pick the former from their 

 bodies and to plant fragments of sponge in their place (see 

 Fig. 8). 



In some forms (especially Galatheidea and Dromiacea) 

 the last two pairs of thoracic limbs are reduced in size and 

 are carried pointing dorsalwards. By means of these 

 upturned appendages (chelate in the Dromiacea) the crabs 

 are able to hold shells and similar objects (in tropical forms, 

 even mangrove leaves) upon their backs. 



It is scarcely necessary to point out that masking and 



