EVOLUTION 12 
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 Herme@a den- 
dritica confine themselves to green seaweeds, whereas 
the red H. bifida is found only on red weeds. Archi- 
doris flammea 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 yisual nerves, and by reflex 
action they are brought into play, s 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 Chamzeleon. 
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. 
Limnea truncatula, which delights to crawl out of 
water on wet mud, is usually so coated over as to 
