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 Spongiae, 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 brigh^ 

 and deep hue, or their rind is of a brilliant colour : 

 we can readily suppose this envelope to have been 



