258 A THEORY OF BIRDS’ NESTS, 
protection has played, in determining the external form 
and colouration, and sometimes even the internal 
structure of animals. 
As illustrating this latter point, I may refer to the 
remarkable hooked, branched, or star-like spicule in 
many sponges, which are believed to have the function 
chiefly, of rendering them unpalatable to other crea- 
tures. The Holothuride or sea-cucumbers possess a 
similar protection, many of them having anchor-shaped 
spicules embedded in their skin, as the Synapta; while 
others (Cuviera squamata) are covered with a hard 
calcareous pavement. Many of these are of a bright 
red or purple colour, and are very conspicuous, while 
the allied Trepang, or Beche-de-mer (Holothuria 
edulis), which is not armed with any such defensive 
weapons, is of a dull sand- or mud-colour, so as hardly 
to be distinguished from the sea bed on which it 
reposes. Many of the smaller marine animals are 
protected by their almost invisible transparency, while 
those that are most brightly coloured will be often 
found to. have a special protection, either in stinging 
tentacles like Physalia, or in a hard calcareous crust, 
as in the star fishes. 
Females of some Groups require and obtain more Pro- 
tection than the Males. 
In the struggle for existence incessantly going on, 
protection or concealment is one of the most general 
and most effectual means of maintaining life; and it is 
by modifications of colour that this protection can be 
