xix 
After this sketch, a general definition of their form 
will scarcely be expected ; for if this be impractica- 
ble as respects one division, what must it be in 
regard to all these animal productions ? 
COLOURS. 
If, notwithstanding the energetic influence of air 
and light upon these organized beings, we may judge 
from the hues that still remain to them in the collec- 
tions, their colours must be varied and brilliant in the 
extreme on their natural sites in the depths of the sea. 
The atmospheric fluids have a rapid influence on 
the colour of the Polypidoms : it is by no means rare 
to see a Sertularia of a brilliant yellow when first 
discovered in the sea, turn to a tarnished brown three 
hours after it has left it. The change sometimes is 
still more rapid ; I have seen Spongias, of a lovely 
lilac when covered with the water, become nearly 
white when exposed a few minutes to the contact of 
air and light. There are however Polypidoms pre- 
served many years in the collections, whose colours 
appear more permanent ; their stems are of a bright 
and deep hue, or their rind is of a brilliant colour : 
we can readily suppose this envelope to have been 
