HYDRA AND CCELENTERATES. 129 
In what part of the aquarium with reference to light 
are they most abundant ? 
Place a hydra in a watch-glass in water and observe 
it more closely with a magnifying glass. What do 
you see ? 
Watch one take a particle of food. 
Notice the foot, the body, the tentacles, the hypo- 
stome, and the mouth. 
What happens when you touch a tentacle ? 
In what tespects does the hydra resemble the 
sponge ? 
How do the two animals differ ? 
Cut a hydra in several pieces and place these in a 
dish in clear water. Watch the pieces for several 
days. 
Place a single budding hydra in a battery-jar with 
water and food. At frequent intervals count the 
number of hydras present. 
Summary of Drawings. (a) A single hydra with 
tentacles showing X IO. 
(6) A hydra with tentacles contracted x Io. 
(c) A single hydra showing buds and branches X Io. 
(2) Several hydras in different positions taken in 
locomotion X Io. 
The Activities of the Hydra. Taking Food. The 
hydra uses the tentacles surrounding the upper end of 
its cylindrical body for the purpose of seizing its prey 
and conveying it tothe mouth. These tentacles are 
covered with minute nettling-cells which paralyze the 
small crustacean as soon as he is seized. These cells 
have within them a very minute coiled thread which is 
uncoiled when the cellis touched. This thread pierces 
the victim which has been unwittingly the cause of the 
uncoiling and poisons the part it pierces. These 
nettling-cells are sometimes called thread-cells or 
lasso-cells. They are characteristic specializations of 
the sub-kingdom Celenterata. 
Digestion. The food after being pushed into the 
