COELENTERATES IN GENERAL 



153 



individuals, or blastostyles. The latter produce medusae by budding, 

 and the medusae in turn produce sperm cells and egg cells. From the 

 fertilized egg cells develops another generation of planulae. The 

 polyps thus represent the asexual generation and the medusae the sexual. 

 Metagenesis is also shown by scyphozoan jelly fishes, the medusae 

 reproducing sexually and the strobila asexually. The hydra does not 

 show metagenesis, because the same individual exhibits both types 

 of reproduction. 



Fig. 71.— a, portion of a hydrozoan coral, the pepper coral {MiUepora sp.), often 

 called the stag-horn coral. Natural' size. B, a portion of a colony with polyps of red 

 jeweler's coral, Corallium rubrum. C, beads made from the skeleton. 



182. Corals.— Corai is a deposit of Ume formed by coelenterate 

 polyps. One type, the pepper coral, or stag-horn coral (Fig. 71A), is 

 distinguished from the rest by the fact that being produced by a simple 

 hydrozoan polyp its mass is relatively continuous and the pits which 

 lodged the hving polyps are simple. On the other hand, most anthozoan 

 corals possess pits which are larger, deeper, and show radial ridges of 

 varying lengths. They are often delicately sculptured, producing a 

 very beautiful effect. Among such corals (Fig. 72) are those known as 

 the elk-horn coral, the brain coral, the rose coral, and the mushroom 

 coral. The organ-pipe ooral and the red, or precious, coral fall in a third 

 group, produced by polyps related to sea fans and sea pens. Coral 



