102 MANUAL OF ZOOLOGY sect. 



protection of the polypes and blastostyles respectively ; 

 but in others (Fig. 47) these protecting structures are 

 absent. The polypes resemble those' of Obelia in all es- 



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sential respects, but differ in the number and arrangement 

 of the tentacles and other minor points. In many medusas 

 are developed from blastostyles as in Obelia, and when 

 fully formed become free. The shape of the medusa 



