CCELENTERATA 



139 



the central cavity has assumed a definite tubular shape 

 (Figs. 79, A; 79, B). 



In the sea anemone the middle layer, or mesoderm, 

 is more highly developed than it is in the jellyfish, and 



Fig. 78.— Diagram of sea anemone. (From Kiugsley, after Emerton.) 



it does not have the jelly-like character. The central 

 cavity is more like that of the hydra except that the 

 elevated hypostome is pushed in and hangs down as an 

 oesophagus inside the animal. It is held in place by 

 partitions having long attached filaments that assist in 

 the digestion of the food (Fig. 79, C). 



Assimilation. — In this group the first step in the de- 

 velopment of the digestive system is taken. The tract 

 may be described as a blind tube with a single opening 



