78 THE WONDER OF LIFE 



Many open-sea animals are transparent ; many of those 

 that live near the surface have beautiful blue and violet 

 colours, well seen in the Siphonophore Velella and in the 

 Gasteropod lanthina. It is possible that there is occasion- 

 ally some adaptiveness in the transparency, though this 

 quality follows for the most part from the lightness of 

 build. It must in some cases make the swimmers or 

 drifters practically invisible. Even in a small bowl of 

 sea- water it is very difficult to see an arrow-worm (Sagitta) 

 or the like, and it is very interesting to watch in a large 

 aquarium how the quite unique Venus's girdle (Cestus 

 veneris), which is at once transparent and iridescent, is 

 conspicuous at one moment a creature of positively daz- 

 zling beauty and invisible the next. Some pelagic 

 fishes, such as a quaint little sea-horse, which live among 

 the Sargasso weed, have the body reddish brown, but the 

 fins, which are spread out in the open water, are a beautiful 

 transparent blue. It is no argument against the theory 

 that transparency is advantageous to point out that it 

 is often of no avail, e.g. when the great Cetacean catches 

 thousands of sea-butterflies in its net. 



There is evidently a considerable intensity of life in 

 some of the Plankton animals, for their movements are 

 practically ceaseless, and their sensory equipment, especi- 

 ally in the way of eyes and balancing organs, is often very 

 remarkable. Many are ' phosphorescent ', such as Nocti- 

 luca, Ctenophores, Pyrosoma, but the vital significance 

 of this remains a riddle. Many move about in shoals, 

 which indicate prolific reproduction and great abundance 

 of food. The numbers are greatest in the colder seas, 

 which is probably due to the fact that at low temperatures 

 growth and development are slowed, the life is drawn out, 



