156 TEXTBOOK OF ZOOLOGY 



outer layer is now known as ectoderm. Further changes involve the 

 secretion of the thin mesogloea which seals the two layers together. 

 In the meanwhile a shell is produced about the outer surface of the 

 embryo, and this encysted body falls from the parent to the bottom. 

 If conditions are favorable for development, it increases in length 

 within the cyst; when it has attained some size it breaks out, after 

 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 used as food by some of 

 the useful fish. The corals are of importance both positively 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. 



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. 



