THE COLORATION OF ANIMALS 63 



Another group, which agrees in colour with the general sur- 

 roundings, is that of the ' glass-animals,' as they have been called, 

 though perhaps ' crystal animals ' is a better term. A great number 

 of simple free-swimming marine forms, and a few fresh-water ones, 

 are quite colourless, and perfectly transparent, or have at most 

 a bluish or greenish tinge, and on this account they are quite 

 invisible as long as they remain in the water. In our lakes there lives 

 a little crustacean about a centimetre in length, of the order of 

 water-fleas (Leptodora hyalina), a mighty hunter among the smallest 

 animals, which swims forward jerkily with its long swimming- 

 appendages, and widely spreads its six pairs of claws, armed with 

 thorny bristles, like a weir basket, to seize its prey. We may have 

 dozens of these in a glass of water without being able to see a single 

 one, even when we hold the glass against the light, for the creatures 

 are crystal-clear and transparent, and have exactly the same refrac- 

 tive power as the water. It requires a very sharp scrutiny and 

 a knowledge of the animals to be able to detect in the water little 

 yellowish stripes, which are the stomachs of the animals filled with 

 food in process of digestion, for which, as we can readily understand, 

 invisibility cannot very well be arranged. If the water be then 

 strained through a fine cloth, a little gelatine-like mass of the bodies 

 of the Leptodora will remain on the sieve. 



A great many of the lower marine animals are equally transparent, 

 and as clear as water; most of the lower Medusee, the ctenophores, 

 various molluscs, the barrel-shaped Salpee, worms, many crustaceans 

 of quite diflferent orders, and above all an enormous number of larvae 

 of the most diverse animal groups. I can remember seeing the sea 

 at the shore at Mentone so full of Salpse, that in every glass of sea- water 

 drawn at random there were many of them, and sometimes a glass 

 held a positive animal soup. But one did not see them in the glass of 

 water, and only those who knew what to look for recognized them by 

 the bluish intestinal sac that lies posteriorly in the invisible body. 

 But when the water was poured oif through a fine net, there remained 

 on the filter a large mass of a crystalline gelatinous substance. 



' It is obvious that this must serve as a protective arrangement, 

 for the animals are not seen by their pursuers ; but it is not an 

 absolute protection, for they have many pursuers who do not wait till 

 they see their prey, but are almost constantly snapping the mouth 

 open and shut, leaving it to chance to bring them their prey. No 

 protective arrangement, however, affords absolute security ; it protects 

 against some enemies, perhaps against many, but never against all. 



But now let us turn to a group of a difierent colouring, the green 



