TWO-LAYERED ANIMALS— PHYLUM COELENTERATA 



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the power to regenerate themselves if cut off. Sometimes a new tentacle 

 will be regenerated when an old one is only injured, and thus a forked 

 tentacle will be formed or two in place of the original one, just as in the 

 myth. This may be seen in the laboratory or may be produced by crush- 

 ing one side of a tentacle without injuring the other side. Also, you 

 may actually produce a two-headed hydra by cutting the oral end down 

 a short distance ; each half of a "head" will regenerate the other half 

 and there will be two complete oral ends attached to a single aboral end. 

 Similar incomplete fissions may occur accidentally in higher animals in 

 the embryonic stage and produce two-headed calves or various degrees of 



Fig. 9.4. A two-headed hydra produced by cutting the body longitudinally at the oral 



end and allowing each half head to regenerate the other half. Also, note that one of 



the tentacles is forked. This was caused by an injury on the side of a tentacle with 



the subsequent regeneration of a portion of a tentacle from the injured region. 



"siamese twins" in human beings. Most of these are unable to live long 

 after birth unless the fission is almost complete, but this is just another 

 example of the similarity of embryos of higher animals with the adults 

 of some of these lower forms. 



Obelia is a colonial organism which in many ways resembles hydra, 

 but in other ways is a more complex form of life. When buds form, a 

 new polyp is produced, but it remains attached to the old individual. 

 Soon a large colony of polyps is produced which may appear as a single 

 individual, but it is actually a colony of many animals. Such colonial 

 animals are called hydroids because of their resemblance to the hydra. 

 Each feeding polyp has a mouth surrounded by a circle of tentacles equipped 

 with tiny batteries of stinging cells. 





