CHAPTER V 
THE C@LENTERATES 
29. General remarks.—This division of the many-celled 
animals includes the jelly-fishes, sea-anemones, and corals. 
A few species live in fresh water, but the majority are con- 
fined to the sea, being found everywhere from the shore- 
line and ocean surface to the most profound depths. 
Adapted to different surroundings and modes of life, they 
constitute a vast assemblage of the most bewildering di- 
versity. In some cases their resemblance to plants is re- 
markable, and the term zoophyte or “ plant animal,” occa- 
sionally applied to them, is the relic of former times when 
naturalists confounded them with plants. Even to-day 
certain species are sometimes collected and preserved as 
seaweeds by the uninformed. 
The general plan on which all celenterates are con- 
structed is a simple sac, in some respects resembling that 
of the lower sponges, yet, since the modes of life of the 
members of the two groups are usually quite unlike, we 
shall find many profound differences between them. 
30. The fresh-water Hydra,—The bodily plan comes out 
most clearly in the Hydra (Fig. 16, A, D), which occurs 
upon the stems and leaves of submerged fresh-water plants 
in this and other countries. Its body, of a green or grayish 
color, according to the species, scarcely ever attains a diam- 
eter greater than that of an ordinary pin nor a length ex- 
ceeding half an inch. One end of the cylindrical organism 
is attached to some foreign object by means of a sticky 
secretion, but as occasion requires it may free itself, and by 
