X) ANIMAL COLOKATKJN. 



A particularly abundant form is Tuhifex rivuloricm, which 

 lives associated in great numbers and partially embedded in 

 mud at the bottom of streams, etc. ; the head end is fixed in 

 the mud, while the tail waves about freely in the water ; these 

 worms form exceedingly conspicuous red patches, which must 

 attract ground-feeding fish. The colour is due to a substance 

 termed htemoglobin dissolved in the blood ; this substance is 

 also found in the blood of the higher animals, and it plays the 

 chief part in respiration ; it is able to absorb from the air, and 

 readily give up to the tissues, oxygen. 



A thickening of the body walls of the worm, or an extensive 

 deposition of pigment, would no doubt render them less visible, 

 but would probably at the same time interfere with the efficacy 

 of the respiratory processes. 



This substance hasmoglobin is a very widely spread respira- 

 tory pigment, but it is not of much importance as giving a 

 colour to the animal except in the group of the Annelids. 



Chlorophyll. 



Another pigment which is of physiological import is chloro- 

 phyll. The colour of all green plants is due to this substance; 

 which is found in the cells of the more superficial tissues. The 

 importance of chlorophyll to the plant is enormous. The actual 

 physiological processes which occur are not yet thoroughly 

 understood, but the result is that in some way or other the 

 living matter— the protoplasm— of the plant is able with the 

 help of the green colouring-matter to split up the carbonic 

 acid of the air into its constituent elements, carbon and oxygen. 

 The carbon combines with the water in the plant to form starch. 

 This process can only go on in the presence of sunlight : if 

 some fragments of a plant that has been exposed to the sun 

 for some time be teazed up with needles, and stained with a 



