THE WONDER OF LIFE 535 



from an adult Convoluta cannot live independently, and yet 

 we know that the Alga Hves freely in the water. The in- 

 vestigators have shown that this is due to the fact that in 

 association with the worm the Algoid cells suffer degenera- 

 tion of their nucleus. Thus the Alga becomes dependent on 

 its partner, which in turn becomes dependent on it, for in 

 course of time the Convoluta ceases to take in food, relying 

 upon the materials worked-up by its partner. 



It should also be noticed that some animals owe their 

 colour directly to their food. Thus some caterpillars are 

 green because of the chlorophyll of the leaves they eat ; 

 and some sea-slugs appear to borrow the pigment of the 

 sponges they browse on. 



If we rank whiteness as a colour, it must be regarded as 

 structural, for it is usually due either to minute gas-bubbles 

 in the cells, as in white hair and feathers, or to minute 

 crystalline spangles, as in many silvery fishes. According 

 to Metschnikoff, the whitening of hairs and feathers in 

 winter is in certain cases due to the activity of phagocytes, 

 which transport the pigment into the skin. He made 

 observations on the Mountain Hare (Le-pus variabilis), on 

 the Willow Grouse {Lagopus albus), on the ptarmigan 

 (Lagopus alpinus), and on a hen which began to turn white, 

 and found the so-called ' chromophagous ' cells actively 

 at work. 



As to the form in which pigment occurs in animals, 

 there is great diversity. It may be precipitated in a non- 

 living layer, Uke the zoonerythrin in the cuticle of crabs 

 and lobsters, shrimps and prawns ; it may be in the form of 

 minute granules in a thick fluid, as in the sepia of cuttle- 

 fishes ; it may be a solid mass, Hke the cochineal of coccus 

 insects ; it may be in the cells of the blood, as in Verte- 



