112 COLOUR IN NATURE chap. 



As to structure, we recognise in each zooecium the 

 firm coating which surrounds and protects the con- 

 tractile polypide. The polypides themselves have a 

 ring of tentacles surrounding the mouth, and a well- 

 developed alimentary canal. The cavity of the 

 zooecium is largely filled up with the so-called 

 funicular tissue, which consists of a network of 

 branching cells, the meshes of the network contain- 

 ing numerous transparent connective -tissue cells, 

 which may be called leucocytes. 



In certain forms, e.g. in Bugula neritina, this 

 funicular tissue is coloured by pigment which varies 

 in tint and gives the colony a purple or yellowish- 

 brown colour. In other cases this tissue is quite 

 colourless. Krukenberg studied the pigments in the 

 case of Bugula neritina, and found that there was in 

 the first place an interesting colouring matter readily 

 soluble in cold water and glycerine, to which it gave 

 a rose-red tint. On account of the presence of this 

 pigment, dying specimens of this species, as was 

 observed by Cohn, coloured the water in which they 

 were found a deep purple. When the purple solu- 

 tions were kept for some time the tint changed to a 

 pale yellow, but the original purple could be restored 

 by shaking with air. The purple solution gave two 

 bands in its spectrum, which disappeared completely 

 when it turned yellow. On account of the readiness 

 with which it could be oxidised and deoxidised, 

 Krukenberg credited the pigment with respiratory 

 importance. Besides the red colouring-matter there 

 is also present in Bugula neritina a yellow pigment 

 apparently of lipochrome nature. 



Besides these observations of Krukenberg's on 



