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Part III. — Eighteenth Anniml Report 



reaches a size of about one inch, passes into the beach-group ; when it 

 has grown to a size of 2^ inches it leaves the beach for the littoral 

 waters just beyond low-water mark, stony or rocky ground ; on arriving 

 at a size of 4 inches it joins the mature group and takes part in 

 the great seasonal migrations. In support of the probability that 

 the crabs of different sizes are restricted to more or less fixed habitats, 

 one has only to consider that, except for this distribution, the crab 

 occupies the same position in the economy of the sea during its whole 

 life. It is a bottom-feeder, and it is apparent that the small crabs 

 would be extremely liable to become the prey of their larger brethren 

 if they occupied the same habitat. And from the fact that the crabs of 

 Groups I. and III. are to be occasionally found with Group II. on the 

 beach, it is very probable that the habitats of the three groups are con- 

 tiguous. The mature Group IY. gives rise to the crab-fishing. 



[Table. 



