66 



COELENTERATA 



as safe harbors for ships do not offset the many dangerous coral 

 reefs that threaten ocean shipping. 



Fig. 29. Whitsunday Island in the South Pacific, an atoll built by corals. (After 



Darwin.) 



Ctenophora. — The Ctenophora (Gr. ktenos, of a comb; phoreo, I 

 bear) are free-swimming marine animals, extremely transparent, 

 and for the most part found in tropical seas, although quite gen- 

 erally distributed. They are called sea walnuts or comb jellies. 



(Figure 30.) 



They are of little importance 

 except as food for other marine 

 animals. The U. S. Bureau of 

 Fisheries has, however, reported the 

 appearance of great numbers of 

 Ctenophores coincident with the 

 disappearance of oyster larvae in 

 Great South Bay. Formerly known 

 as destructive to molluscan larvae, 

 there have been a number of years, 

 1917, 1921, 1927, when heavy 

 " sets " of young oysters were lost 

 during the month of June. Prob- 

 ably certain temperature conditions 

 were responsible for the appearance 

 of the Ctenophores at a date earlier 

 than usual, and at a time when they could do damage to young 

 oysters. It is of course also barely possible that the temperature 

 changes were injurious to the oysters, and that the injury done by 

 Ctenophores was correspondingly less. 



Fig. 30. Mnemiopsis, a Ctenophore 

 (Courtesy of T. C, Nelson.) 



