322 ESSENTIALS OF ZOOLOGY 



which tentacles and a mouth appear at one end, while the enteron 

 develops within the endoderm. This individual steadily grows and 

 soon attains adult condition. When the zygote is formed in the 

 fall, the embryo does not emerge from the cyst until spring. 



Regeneration 



As is the case in many invertebrate and a few vertebrate animals, 

 Hydra is able to replace mutilated parts or an entire animal from a 

 portion of one. Complete animals may be formed from very small 

 pieces (% mm. in diameter) of a hydra. This process is known as 

 regeneration, and while it is not normally a method of reproduction 

 or multiplication, it is of great advantage to the animal. This phe- 

 nomenon was first discovered in animals from studies on Hydra in 

 1744 by Trembly. 



Economic Relations of the Phylum 



The entire group is not worth much in dollars and cents to man 

 directly. A number of different ones are made use of as food by 

 some of the useful fish. The corals are of importance both posi- 

 tively and negatively. Many of them are valuable as ornaments, 

 while the large coral reefs are very costly to navigation of marine 

 waters. Many corals are quarried for building stone, and in some 

 instances they protect the shore from being washed by the waves. 



f 



Phylogenetic Advances of Coelenterates 



(1) Definite organization of diploblastic condition; (2) well- 

 defined gastrovascular cavity with one opening, the mouth; (3) 

 presence of tentacles with (4) nematocysts or sting-bodies ; (5) 

 continuance of sexual reproduction; (6) distinct radial symmetry; 

 and (7) a nerve net. 



References 



Hegner, E. W.: Invertebrate Zoology, New York, 1933, The Macmillan Company. 

 Hickson, S. J.: Coelenterata, Cambridge Natural History, New York, 1906, The 



Macmillan Company. 

 Ward, H. B., and Whipple, George C: Fresh-Water Biology, New York, 1918, 



John Wiley & Sons, Inc. 



