122 ANIMAL COLORATION. 



If, however, a quantity of sea-water be strained through a 

 muslin bag, a gelatinous mass will be left behind. On a closer 

 examination this proves to consist entirely of the bodies of 

 innumerable animals, so perfectly transparent as to be prac- 

 tically invisible when floating singly. So abundant in every 

 latitude is life in the surface waters, it has been calculated 

 that there are eight hundred tons of organisms to every 

 square mile on our coasts. At night the phosphorescence 

 of these organisms renders their presence obvious. 



The surface fauna of the oceans comprises representatives of 

 all the groups of invertebrate animals — chiefly, perhaps, larvte, 

 but also numerous adult forms ; and these all agree in the 

 entire absence of pigment, or in having but a small amount; 

 and when colour is present it seems to suit their special cir- 

 cumstances. The Salpfe, for example, are colourless and 

 transjiarent, with the exception of a small portion of the 

 alimentary canal ; this has a brownish-yellow colour, and 

 suggests a fragment of floating seaweed. The ultramarine 

 blue of the floating Velella has been compared to the blue of 

 the sea. Among pelagic fish it is common to find the upper 

 surface dark-coloured and the lower surface white, so that the 

 animal is inconspicuous when seen either from above or below. 



The transparency of the greater part of the pelagic fauna is, 

 so far as our senses are concerned, an undoubted protection to 

 them, rendering them, of course, invisible. Inasmuch as many 

 of these organisms have close relations which are brightly and 

 variously coloured, it is believed that natural selection has 

 effected the change from a coloured to a colourless condition ; 

 the sujiposed advantage is naturally the concealment afforded 

 by this change. The Ascidians that live attached to stones or 

 weed are generally brilliantly coloured, while the pelagic forms 

 sucli as tlie Salpa; are nearly perfectly transjjarent. 



