CLASS HTDEOZOA. 



271 



length ; yet so largely composed of water that, when dried, 

 nothing is left but a film scarcely sufficient to stain a sheet 

 of white paper. They are voracious, feeding on smaiU fishes, 

 crustaceans, and even on 

 their own species. Mere 

 transparent masses of 

 jelly, and only visible 

 because of their brilliant 

 colors, they move 

 through the water rapid- 

 ly, and lasso their prey 

 with great precision. 



The Siphonophorse 

 (tube-bearers) are free 

 swimmers and character- 

 ized by having one or 

 more large air- sacs, 

 giving great buoyancy. 



The Hydroidae (hy- 

 dra-form) include the 

 compound Sertulariaus 

 (see Pig. 393) and the 

 simple Hydras. The 

 former resemble minute 

 ■ferns, and are often mis- 

 taken for sea- weed. Some 

 species of Hydra are 

 found in fresh water, 

 attached to the stems of 

 plants. When contracted, they are a little mass of green 

 jelly. Expanded, their body is a tube, half an inch long, 

 not larger than sewing-thread.* By the sucker at the base 



PhyscUia ufriffUlus^ Portuguese Man-of-war, 

 Tropical Atlantic. 



* It may even be tnmed inside out like a glove — what was the outer surface 

 becoming a stomach and beginning to digest the food, and what was the intestinal 

 canal taking np the functions of absorption and respiration. Sever a Hydra''s 

 stomach, and the mouth will still keep taking in food, though there is nothing to 

 hold it until the digestive organ grows again. " The same Polyp may be successively 

 inverted, cut in sections, and tamed back again without serious harm." 



