ZOOPHYTES, JELLY-FISH 
AND CORALS 
(Coelenterata). 
The sub-kingdom of the Coelenteratcs includes a variety 
of animals, tlie Freshwater Polyp, the Zoophytes, Jeily-Fish, 
Sea-Anemones, Corals, Sea-Fans and Sea Pens. 
Their structure is best explained on that of the simplest 
member of the group, the Freshwater Polyp Hydra, which U m 
common in Europe and has been found iit many other parts of 
the world as well, though it has not yet been recorded from 
Asia. 
Hydra has a slender tubuSar body, closed at the lower 
end, with a mouth at the upper end and a series of tentacles, 
generally six to eight, arranged around the mouth. The lower 
end is disk-shaped and serves for the attachment of the animal 
to water weeds and other objects. The body-wall consists 
of two layers of cells, an outer one, the ectoderm, which is 
protective, sensory and reproductive, and an inner one, the 
endoderm, which Is digestive. The cavity enclosed by the 
body-wall is called the coelenteron. Both body and tentacles 
are highly contractile. When fully expanded the two together 
may measure as much as an inch in length. When contracted 
they form roundish knobs barely 1 ,16 inch in size. Hydra is 
carnivorous and paralyses and kiJls its prey by means of its 
tentacles. Imbedded into the ectoderm of I he tentacles, and to 
a lesser degree into that of the body, is a curious kind of cells, 
called thread-cells or nematocysts. Each of these cells contains 
a spiraliy coiled up thread which is provided with barbed hooks 
and some poisonous substance. When the Hydra becomes 
irritated, e.g. by a passing animal, the threacis become discharged 
and fix themselves into the body of its prey, paralyzing and 
killing it. All other Coelenterates possess similar thread-cells, 
and any one who has handled living Zoophytes and Jelly-Fish 
will know the stinging sensation caused by them. 
Hydra shows in its tubular body and the arrangement of 
the tentacles around the mouth a radial symmetry of structure 
