CHAPTER XI 
CCELENTERATES. HYDRA-LIKE ANIMALS 
138. Ccelenterates. — The Coelenterates (se-len'te-rats : 
Greek, koilos, hollow; enteron, intestine) are simple Metazoa, 
a little higher in development than the sponges. In this 
group are hydras (hi'dras), hydroids (hl'droids), jelly-fishes, 
sea-anemones (a-nem'o-nez), sea-fans, and corals. 
139. Structure of Hydra. — The hydra is an interesting 
fresh-water animal about a quarter of an inch in length. Its 
body is shaped like a little cylindrical bag with only one open¬ 
ing, the mouth, which is surrounded by a few, usually six, 
delicate, thread-like arms called tentacles (ten'ta-kls). The 
body is composed of three layers, the outer layer, ectoderm; 
the middle layer, the mesoglea (mes-o-gle'a: Greek, mesos , 
middle; gloios, glutinous substance); and the inner layer, 
endoderm. 
Each layer does some particular work for which the others 
are not fitted. For example, the outer layer contains cells 
which are especially sensitive to stimuli and many modified 
muscle cells that enable the animal to move about. The 
inner layer contains cells provided with flagella which catch 
the food particles for the inner cells to digest. The muscular 
action of the outer layer moves the entire animal. The sensi¬ 
tive cells enable the animal to recognize its prey. The food 
digested by the inner layer is used by all the cells of the body. 
Thus we see an advance in the division of labor over that 
shown in the sponge. We shall observe a still greater in¬ 
crease in division of labor as we study higher animals. 
Tentacles are hollow, finger-like branches connected with 
the body cavity. They are provided with stinging cells 
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