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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