28 EVOLUTION OF LIFE. 
upside down, and swims off, stomach hanging downward 
from an umbrella or bell-shaped body ; we would then have 
a jelly-fish. 
HYDROZOA. 
Every one who has visited the sea-shore must have had 
his attention called to the jelly-fishes (Fig. 20), as they 
floated along by means of the pulsations of their disc-bear- 
ing bodies, the animal looking somewhat like an umbrella, 
and he remembers well the sensations suffered while bath- 
ing when his skin came in contact with the long streamers 
floating about, which are so characteristic of these organ- 
isms. The stinging is due to a poison which is contained 
in vesicles situated in the skin, often millions in number. 
In the beautiful Blue Physalia, known to sailors as the 
Portuguese man-of-war, this poison is so powerful that it 
has been known to cause death. The jelly-fishes and Anem- 
ones alike possess these poison-cells; but the Sponges, 
although having the rudimentary canal system, are devoid 
of the stinging structures. The most simple example of 
the Hydrozoa is our common Green Hydra (Fig. 18), so 
called from the fact that when cut in pieces each piece 
becomes a new individual. It looks to the naked eye like 
a piece of green silk thread. When magnified, it is seen 
to be a simple tube, the digestive cavity and general cavity 
of the tube being the same; its mouth is surrounded by a 
circle of arms or tentacles, by means of which it seizes its 
prey, paralyzing or destroying it by the poison just spoken 
of The importance of the Hydra, as part of the evidence 
for the evolution of life, may be seen in the development of 
the so-called Hydroid jelly-fishes, such as the Campanularia 
and Sertularia (Fig. 19), which look like little trees covered 
with flowers. The branches of these tree-like beings are 
little tubes, in which the flowers (Hydroid polyps) live; tlie 
tubes are all connected, so the colony has a common 
SOA" OLISDA DD 
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