CHAPTER V 



OCELENTERATES. HYDRA-LIKE ANIMALS 



48. Ccelenterates. — -The Coelenterates (se-len'te-rats 

 Greek, koilos, hollow ; enter on, intestine) are simple 

 metazoa, a little higher in development than the sponges. 

 In the group are hydras (hl'dras), hydroids (hi'droids), 

 jelly-fishes, sea-anemone 



(a-nem'o-ne), sea-fans, 

 and corals. 



49. Structure of Hydra. 

 — The hydra is an in- 

 teresting fresh water 

 animal about a quarter 

 of an inch in length. 

 Its body is shaped like 

 a little cylindrical bag 

 with only one opening, 

 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 : FlGURE 60 . - m.crophotographs of 

 Greek, mesos, middle ; Hydra. 



G3 



