V COLOUR-PHENOMENA IN WORMS 103 



green of Eulalia viridis, of Sabella, and of many 

 others, the pink of Terebella and Euchonc rosea, and 

 the bright red filaments of Cirratulus, are examples 

 of the bright tints which are so widely spread in the 

 group. 



Before passing on to consider what is known as 

 to the nature of the pigments, we may note the 

 different ways in which the external coloration of 

 the Annelids is produced. In the smallest and 

 simplest forms there is no pigment either in the 

 blood or tissues, and the animals are therefore 

 transparent and colourless. But when they are 

 herbivorous, as is often the case, the contents of the 

 food-canal may give them a green colour. The next 

 stage may be described as the condition when the 

 tissues retain their transparency and lack of pigment, 

 but the coloured blood shines through the thin 

 body wall, and gives the animals a distinct and often 

 bright colour. This is well seen in the bright red 

 Tubifex of ditches, the green Sabella in its sand- 

 tubes on the seashore, and others. Again, the 

 tissues may be devoid of pigment, but may be too 

 opaque to allow the blood-pigment to shine through 

 except in certain regions. Thus, for example, in 

 Gordiodrilus tenuis the general colour is a cream 

 white produced by the coelomic corpuscles, but the 

 body is marked by longitudinal red stripes produced 

 by the shining through of the larger blood-vessels. 

 Generally, however, we find that forms in which the 

 .tissues are too opaque to allow the blood to be 

 directly visible not infrequently develop pigment 

 in the tissues. Thus the common earthworm con- 

 tains a certain amount of pigment scattered among 



