CHAPTER VIII 



CNIDARIA 

 HYDROZOA 



A FRESHWATER POLYP. HYDRA 



This small animal is of general although sporadic distribu- 

 tion throughout the country. It frequents ponds and streams 

 abounding in plant life, and is best obtained by collecting water 

 together with water plants, sticks, and other objects from sev- 

 eral such places, and allowing it to stand in glass jars. The 

 polyps will, if present, be seen, after an interval of some time, 

 attached to the stems or leaves of the plants, or to the sides of 

 the jar. They may be kept indefinitely in aquaria of this sort 

 and will usually multiply rapidly. 



Hydra is a slender, tubular animal from one-eighth to one- 

 half an inch in length ; it attaches itself by one end to some 

 stationary object and projects pendant in the water; at the 

 other end is the mouth, surrounded by from four to eleven long 

 thread-like tentacles. It does not attach itself permanently to 

 one place, but can crawl about or swim slowly through the 

 water. Its food consists largely of small crustaceans which it 

 kills or paralyzes with the peculiar stinging organs called nettle 

 cells, located principally in the tentacles. 



Study the animals first, if possible, without disturbing them 

 and with the aid of a hand lens. Note the extreme contractility 

 of the body. Look for individuals with distended bodies. 

 These have just swallowed prey. Look for budding individ- 

 uals ; budding is one method of reproduction. Observe the 



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