i86 



THE BOOK OF NATURE STUDY 



probably increase their power of flotation. Many, such as the 

 Medusoids or swimming-bells, the arrow-worms (Sagitta), the sea- 

 butterflies (pelagic Gasteropods), are as transparent as glass ; 

 many, such as some of the common jelly-fishes, have a beautiful 

 blue colour; some of the Ctenophores, notably Venus's girdle, 



FIG. 108. A typical jelly-fish (Chrysaora\ occasionally found off British coasts. The 

 mouth is in the centre below the umbrella-like disc, and is guarded by four long sinuous 

 lips. Around the margin of the disc there are two dozen long tentacles and eight 

 sense organs. Radial canals are seen passing outwards from the central stomach. 



show a fine iridescence as they move, wafted along by the lashing 

 of multitudinous ciliated combs ; not a few, such as the compound 

 tunicate Pyrosoma, are brilliantly " phosphorescent." 



The Plankton is of great practical importance, because it 

 forms the basal food supply of many fishes, and changes in its 

 distribution must be followed by changes in the distribution of 



