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 spiculaa in 

 many sponges, which are believed to have the function 

 chiefly, of rendering them unpalatable to other crea- 

 tures. The Holothuridaa 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 Trepangj 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 



