EVOLUTION 121 



Thus Ovula and Pedicularia are yellow or red accord- 

 ing to the colour of the corals on which they occur. 



The phenomenon is common enough among the 

 Nudibranchs. Green specimens of Hevmcea den- 

 dritica confine themselves to green seaweeds, whereas 

 the red H. bifida is found only on red weeds. Archi- 

 doris fiammea closely assimilates in hue the bright 

 red sponges on which it feeds. Endless similar 

 instances are, in fact, afforded by this group. 



Pelagic species of Mollusca are generally trans- 

 parent and colourless, unless tinged with blue. 



Protective coloration is, however, carried to its 

 greatest perfection in the Dibranchiate Cuttlefishes. 

 Scattered over the skin of these animals are pigment 

 cells containing different colours which become 

 visible on the expansion of the cell by the muscle 

 fibres with which each is furnished. Their control 

 is connected with the visual nerves, and by reflex 

 action they are brought into play, so that the creature 

 is able more or less voluntarily to adapt the colour 

 of its body to its immediate surroundings after the 

 more familiar example of the Chamseleon. 



More frequently, perhaps, protection is as it were 

 adventitiously afforded by the shell becoming coated 

 over with extraneous matter. Many of the land 

 shells, especially those living among the roots of 

 plants, tend to become coated over with earth. 

 Lirnncea tnmcatula, which delights to crawl out of 

 water on wet mud, is usually so coated over as to 



