PHYLUM CGELENTERATA 



171 



group of experiments dealt with the control of the rhythmical 

 pulsation of the umbrella. There are eight nerve centers 

 around the margin of a jellyfish; these control the pulsa- 



B 



HYDROZOA SCYPHOZOA ANTHOZOA 



FIG. 71. Life histories of typical coelenterates. A, zygote; B, ciliated 

 planula; C, young polyp; D, mature polyp; E, medusa. In the Hydrozoa all 

 stages usually appear in a well developed condition; in the Scyphozoa the polyp 

 is always small and unimportant; the Anthozoa never have a medusa stage. 



tion and all are correlated so that the area dominated by 

 each beats synchronously with other regions. If all but 



