TYPES OF CELENTERA—HYDRA. 143 
threads is mechanical and chemical. They fix, e.g. by the stilets, into 
the victim, and the secretion poisons the wound, paralysing or killing 
small animals, and sometimes acting as a solvent. Many seem to be 
prehensile threads rather than weapons, 
.TyPES OF C@LENTERA 
first Type—Hynra, a simple representative of the 
Class Hyprozoa 
General life. —The genus 7ydra—cosmopolitan, like many 
other small fresh-water animals—is represented by several 
species, e.g. the green Hydra viridis, the brownish H. oligactis 
or fusca, and the orange ZH. vulgarts 
or grisea, widely distributed in fresh 
water. They are among the simplest 
of Ccelentera, for the body is but a 
two-layered tube, with a crown of 
(6-10) hollow tentacles around the 
mouth, and with no organs except 
those concerned in reproduction. 
The body is usually fixed by its base 
to some aquatic plant, often to the l 
lower surface of a duckweed. It @ff 
may measure 4~4 inch in length, but 
it is as thin as a needle, and contracts ue Prien Saree 
into a minute knob. — After Greene, 
The animal sways its. body and 
tentacles in the water, and it can also : 
loosen its base, lift itself up by its tentacles, stand on 
its head, or creep by looping movements. According 
to some observers, its movements are helped by fine 
pointed pseudopodia protruded from the ectoderm cells 
of the tentacles and base, and by threads ejected from 
large cylindrical stinging cells. Usually, however, the Hydra 
prefers a quiet life. It feeds on small animals, which are 
paralysed or killed by stinging cells on the tentacles, and 
are swept into the tubular cavity of the body by the action 
of flagella on the internal cells. Sometimes animals as 
large as water-fleas (e.g. Daphnia) are caught, and the 
ffydra may sometimes be seen struggling fiercely with 
a small Annelid worm (Zwudzfex). Tpfusorians (Euplotes, 
ov., Ovary; Z., testes. 
