PHYLUM II. CCELENTERATA. 



The Coelenterata and the Echinoderma were formerly 

 united into a group Radiata, the basis of association being 

 the radiate type of structure so noticeable in a starfish or a 

 coral. Later studies showed that these two divisions had 

 very few points in common, and that the differences be- 

 tween them were very great. 



In the Coelenterates there is but a single opening into 

 the digestive tract, which thus serves at once for mouth 

 and vent. Through it all food enters, and all indigestible 

 portions are cast out. The mouth connects with the 

 digestive tract, which extends to all parts of the body, so 

 that the food is brought close to every portion, there 

 being no circulatory apparatus. There is no body-cavity 

 distinct from the digestive tract. The wall of the body is 

 but two layers of cells in thickness, with between them 

 a third laver, sometimes thin and without cells, some- 



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times thick and gelatinous, with scattered cells in the 

 jelly. This third layer is called the supporting layer, or 

 mesoglcea. Around the body, frequently close to the 

 mouth, is a circle of tentacles, and on these abound some 

 structures which need a slight description the nettle- 

 cells. 



These nettle-cells, fig. 12, are small bodies which occur 

 all over the body, but are especially numerous upon 



the tentacles. Each is in reality a sac, one end of 



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