CHAPTER IV 



COELENTERATA 



The Coelenterata (Gr. koilosy hollow; enteron^ Intestine) are 

 aquatic, and are for the most part marine, some developing into 

 enormous colonies. Their bodies, especially the tentacles, bear 

 nematocysts or " thread cells," structures not found in other Phyla, 

 which are used for offense and defense. The corals, which secrete 

 hard exoskeletons, are the most significant economically. 



Classification 



Class L Hydrozoa {Gr. hudra, wa.t€:r serpent; zoon, animal). They 

 include fresh water hydra, marine hydroids, small jelly 

 fishes (Gonionemus), and some stony corals. 



Class II. Scyphozoa (Gr. skuphos, cup; zoon^ animal). They in- 

 clude most of the large jelly fishes (Aurelia). 



Class III. Anthozoa or Actinozoa (Gr. anthos, a flower; zoon, 

 animal). Include sea anemones, most stony corals, sea 

 fans, sea pens. 



Characteristics 



1. Radially symmetrical. 



2. They have two cellular layers, ectoderm and endoderm, with a 



non-cellular mesoglea, a jelly-like substance, between. 



3. They have a hollow body, the central space being called the 



gastro vascular cavity or coelenteron. This may be very 

 small (Aurelia). 



4. They have stinging cells or nematocysts. 



Natural History 



Class I. Hydrozoa. Hydra. (Figure 21.) — Because it is so 

 easy to collect and keep in the laboratory, many zoologists include 

 Hydra in the first course in zoology. It is from 1/16 of an inch to 

 3/4 of an inch long, and lives in fresh water attached by one end, but 

 is able to move about. The body is a tube cylindrical in shape, with 



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