ADAPTATION 



241 



Internal parasites include many protozoa, flatworras and round- 

 worms, Crustacea and insecta. Such forms are, of course, the 

 most extremely modified. External parasites are chiefly arachnida 

 (mites and ticks), and insects. The latter are often adapted in 

 texture, vestiture and structure to aid locomotion on the body 



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Fig. 146. — A hermit crab occupying the shell of a Gasteropod mollusc which 

 is almost concealed by four sea anemones, ac, ac' are acontia of the anem- 

 ones; sh, shell of Gasteropod. (From Parker and Haswell, after Andres.) 



of the host and to resist capture, but they retain all of the struc- 

 tures of independent organisms and must be looked upon as 

 highly specialized rather than as degenerate creatures. 



Coloration and Mimicry. One of the most remarkable phases of 

 adaptation to the organic environment is coloration and mimicry. 

 The complexity of such adaptations is well indicated by the fact 

 that they can be classified under the following diverse heads: 



