AGGRESSIVE RESEMBLANCES, ETC. 77 



nests are often similarly concealed ; the lichen-covered 

 nest of the chaffinch is an obvious instance. 



The well-known cases of Caddice- worms {Trichop- 

 tera) are partly for concealment and partly for defence ; 

 they are built of grains of sand, small shells (often 

 alive), vegetable fragments, — in fact, of any suitable 

 objects which are abundant at the bottom of the 

 stream in which they happen to be. 



Some of the best examples are to be found among 

 marine animals. Certain sea-urchins cover themselves 

 so completely with pebbles, bits of rock, shell, &c., that 

 one can see nothing but a little heap of stones. 



Many marine mollusca have the same habits, 

 accumulating sand upon the surface of the shell or 

 allowing a dense growth of algae to cover them.^ The 

 best example of the kind was shown me by Professor 

 C. Stewart, and is all the more interesting because of 

 the transition observed in the habits of different species 

 of the same genus, Xenophora, Many of these gastro- 

 pods include pieces of shell, rock, coral, &c., in the 

 edge of the growing shell. The effect is probably to 

 obscure the junction between the shell and the surface 

 on which it rests, and thus to assist in rendering the 

 organism difficult of detection. Thus the growth of 

 the shell may be traced by a spiral line of included 

 fragments {X. calculifera) . In X. Solaris the habit is 

 only maintained during the early stages of growth, 



' E. S. Morse: Proc. Boston Soc. Nat. Hist. vol. xiv. April 5, 

 1871, p. 7. 



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