98 THE BIOLOGY OF THE SEA-SHORE 



plant exposed to sunlight are frequently much lighter in hue 

 than the more sheltered portions, which are usually dark 

 brown or olive." He has found that the darker varieties 

 of L. ohtusata are seldom found upon the surface of the algal 

 masses, and that the greatest resemblance is between light 

 brown and yellow shelled varieties and the vesicles of 

 F. vesiculostis, and between the colour of the frond and the 

 ordinary brown and olive shells. But yellow shells may be 

 found at times on the dark stems, and are then very con- 

 spicuous. Many shells which, when seen dry, appear out 

 of harmony, are not so in reality when wet. 



In considering the resemblance which so frequently 

 occurs between associated forms, the possibility of this 

 being of a convergent rather than a protective nature must 

 be borne in mind. Thus, says Eliot (1910), whatever the 

 causes which give a certain sponge a particular appearance 

 they must also affect the Dorid which lives on the sponge, 

 at least so far as they are external and concerned with water, 

 temperature, and general surroundings. Hence also the 

 resemblance between certain Holothuroids {Psoitis spp.),Tur- 

 bellarians (especially Polyclads with tentacles), and molluscs 

 such as the Elysiidae. 



Warning Coloration. — In sharp contrast to the cases we 

 have just been discussing are those in which the coloration 

 of a particular animal has the effect of rendering it extremely 

 conspicuous. Frequently this vivid coloration is associ- 

 ated with unpalatability or with qualities which render the 

 animal dangerous to eat. For instance, each of the in- 

 tense blue patches which form a ring beneath the tentacles 

 in Actinia equina is associated with a battery of nematocysts. 

 It is therefore concluded that the vivid coloration asso- 

 ciated with the unpleasant taste would, after a few unfor- 

 tunate experiences on the part of an enemy, leave an impress 

 on the nervous system of the latter sufficient to deter it 

 from making fresh attacks. The colours would thus come 

 to have effectively a warning significance. Poulton (1890) 

 quotes an interesting case investigated by Garstang, viz. 

 that of a Terebellid worm, Polycirrus aurantiacus. Unlike 



