Chap. 23 COELENTERATES SIMPLE MULTICELLULAR ANIMALS 485 



medusa after another; the oldest one at the end the first to separate and swim 

 away (Fig. 23.15). 



Class Anthozoa 



Sea anemones and true corals. These are the fleshy sea anemones and the 

 limestone secreting corals. All are polyps, solitary or colonial with no medusa 

 stage. They are distinguished from hydrozoan polyps by the vertical partitions 

 or septa which partially divide the gastrovascular cavity into alcoves opening 

 into a central space below the gullet. 



Sea-anemone — A Representative Anthozoan. Metridium marginatum is the 

 common sea anemone which attaches itself to wharf piles and gathers by 



Fig. 23.15. An underwater photograph of living polyps of Aurelia. Polyps 

 (about one-half-inch long) of Aurelia aiirita, growing on a hollowed rock. Jelly- 

 fishes (medusae) are being formed by the transverse division of the polyps. A 

 young jellyfish (ephyra stage) has just separated from a polyp and is swimmmg 

 into open water. (Photographed from life by Douglas P. Wilson, Marme Biologi- 

 cal Laboratory, Plymouth, England.) 



