174 THE FLOATING ANIMALS 



shore. A few live deep down in the very cold zones of 

 the oceans. 



The Siphonophores (Figs. 96, 98 to 100) are closely 

 related to the Hydromedusae, but most of them have 

 not the appearance of a jelly-fish. Although some are 

 fairly simple in their construction, others are extremely 

 complicated. They usually have the appearance of 

 a colony of animals composed of different kinds of 

 individuals. 



This group contains some of the most beautiful, the 

 most delicate, and the most fascinating animals that 

 live in the sea. They are typically oceanic animals, 

 and keep afloat by the aid of air sacs or oil globules. 

 Nearly all swim, often at a good pace, by means of 

 special organs called " swimming bells." As a rule, they 

 keep below the surface of the sea, and a few have been 

 taken at a very great depth. They are found all over 

 the world, even under the ice off the Antarctic con- 

 tinent, and the species have usually a very wide geo- 

 graphical range. 



Their life-history is very complicated, and in the 

 course of development they pass through stages which 

 are not at all like each other, either in structure or 

 general appearance. 



To the Siphonophores belongs the well-known Portu- 

 guese man-of-war (Physalia, Fig. 98). This is the 

 largest Siphonophore, and it floats by the aid of an 

 air sac at the surface of the sea. Its stinging power is 

 tremendous, producing a maddening pain which lasts 

 for hours. Another common form is the little blue 

 Velella (Fig. 99), which also floats at the surface, and 

 is not infrequently blown ashore. 



Adult sea-anemones (Actiniaria) are rarely found in 

 Plankton ; but larvae of anemones which eventually 

 affix themselves to or burrow in the bottom, are fairly 



