CCELENTERATA. 



The Ccelenterata 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 the two divisions had 

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

 tween them were very great. Remembering the account of 

 the Echinoderms, the following facts regarding the Ccelen- 

 terata will have significance : 



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 por- 

 tions 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. Around the 

 body, usually 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 are small bodies which 

 occur all over the body, but are especially 

 numerous upon the tentacles. Each is in 

 FIG. 142. A dis- reality a sac, one end of which is drawn 

 thffh^e^coliS out into a long and slender tube, coiled up 



around the cell. inside Qf the ^^ Thege ne ttle-Cells Can 



be " discharged" by the animal, and this discharge consists 



310 



