82 ZOOLOGY. 
up in cells around their tentacles, or in some part of the outer layer of the 
body, and are darted out to wound and sting to death any worm or other 
small animal that may come within their reach. The Ccelenterata are all 
aquatic animals, most of them marine. Some are attached by a base to a 
rock or other fixed object, the mouth being at the opposite extremity, while 
others swim freely in the water. Many are compound animals, increasing 
by gemmation, the common stem branching in-very various and beautiful 
forms in the different species, and the individuals—if they may be so 
termed—which compose the community, being arranged with as perfect 
regularity as the flowers of any plant. From their plant-like appearance, 
many of them have been called Zoophytes,! and some have, for a similar 
reason, received the popular names of Animal-flowers and Sea-anemones, 
From the circle of tentacles surrounding the mouth, many are termed 
polypes or polypi.2. The common stem of many of the compound species 
is protected by a horny or calcareous secretion, which gives it strength to 
resist the action of waves and currents, and having a minute cup or cell 
for each polype to retire into upon the approach of danger. Coral reefs 
are formed by the calcareous secretions of polypes, which remain as stony 
masses after the polypes have died, and afford a foundation on which 
other polypes may proceed to build. Great tracts of country and many 
whole islands of the Pacific Ocean consist of coral rock, the work of these 
tiny and apparently insignificant animals. 
The Hydra.—The polypes of the genus Hydra may be regarded as the 
type of one of the chief groups of Coelenterata. They are inhabitants of 
fresh water, and may often be found adhering to stones, submerged leaves, 
or twigs. When not looking out for its prey, the Hydra has the appear- 
ance of a small gelatinous button ; when active, it extends itself into the 
form of a hollow cylinder, less than an inch long, with a circle of slender 
<=» | tentacles round the mouth (a, fig. 66, represents a Hydra 
‘| in this state). The body may be described as consisting 
ae a merely of an outside skin and the lining of the cavity that 
traverses its whole length from the mouth. This cavity is 
all the digestive apparatus the Hydra has; indeed, its 
structure is so simple, that it may be turned outside in © 
without being destroyed, for the outer layer just adapts 
itself to do the duty of the inner, and the inner to do 
- that of the outer. 
It will be noticed that on the side of the Hydra in the figure, there is a 
little one growing. It is thus that the young are produced by gemmation. 
A wart-like knob begins to grow; and as it grows it becomes more and 
more like the parent animal, till at last it becomes contracted where it is 

1 From Greek zd0n, an animal, and phyton, a plant. 
2 From Greek polys, many, and pous, a foot. 
