112 
COLOUR IN NATURE 
CHAP. 
As to structure, we recognise in each zocecium 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, eg. 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 
