C(ELENTERATA 211 



cavity, serving as a stomach and having a single orifice. Relative 

 They have therefore the form of a vase or bottle, and to ofoeiente- 

 that extent resemble the sponges, though the resem- ratestruc- 

 blance is wholly superficial and represents no commu- 

 nity of function or descent. In the other Metazoa, be- 

 ginning with the echinoderms and worms, there appears 

 a second body cavity, the ccelom, between the intestine, 

 or stomach, and the body wall. In their vaselike form 

 with a single cavity, the ccelenterates thus stand at the 

 base of the metazoan series, and in a sense we may say 

 that a jellyfish is less like a sea urchin than the latter is 

 like a man. 



When we have once grasped the essential features of 

 the ccelenterate structure, it is not difficult to detect 

 them in the most diverse members of the group. As the 

 position of the animal differs according to the species, or 

 even in the same species at different periods of life, we 

 do not speak of the upper and lower surfaces, but of the 

 oral and aboral sides. The oral side is that which ex- The oral 

 hibits the mouth opening, and the aboral that opposite surfaced 

 to it. Thus in a sea anemone the upper side is oral, and 

 the mouth is directed upward. In a jellyfish the lower 

 side is oral, and the upper corresponds to the base of the 

 sea anemone. 



3. The ccelenterates possess radial symmetry, in the Radial sym- 

 manner of a flower. This early attracted the attention metry 

 of naturalists ; hence the name "sea anemone," and the 

 scientific term Anthozoa (flower animals), applied to the 

 great group including the sea anemones and most of 

 the coral animals. Others form plantlike colonies, and 

 were in some cases originally described as seaweeds. 

 Such are termed zoophytes, the name meaning in Greek 

 "animal plants." The radial symmetry of the ccelente- 

 rate is said to be primitive, whereas that of the echino- 



