PHYLUM COELENTERATA (CNmARIA). SIMPLE TISSUE ANIMALS 



127 



Figure 65. A fringing reef. Two Island, Pacific Ocean. The light streak around the island is 

 coral, submerged in very shallow water. The gray area outside of that is also coral but in 

 deeper water, and the black area outside of that is very deep sea water. This is one of the 

 many islands which were used by the armed forces during World War II. (Courtesy of Major 

 Dennis G. Cooper.) 



lagoon. There are many of these in the 

 mid-Pacific with lagoons of a few hundred 

 yards to miles in diameter. The atoll of 

 Bikini, of atomic and hydrogen bomb test 

 fame, has a lagoon area of 280 square miles 

 and a land area of only 2.87 square miles. 

 Wake Island and Tarawa are atolls which 

 figured prominently in World War II. 



The horseshoe atoll of West Texas is well 

 known as a prolific source of oil production. 

 It is now buried under thousands of feet of 

 rocks and existed as an atoll in the shallow 

 seas which covered west Texas many mil- 

 lions of years ago. The horseshoe atoll is the 

 largest limestone petroleum reservoir in 

 North America. It is from 70 to 90 miles 

 across and as much as 3000 feet thick. More 

 than 5000 wells have been drilled into the 

 reef mass and over 300,000,000 barrels of 

 oil have been produced to date. 



Barrier reefs and atolls have been built 

 by the epidermal cells of countless numbers 

 of small polyps, each one secreting its cup- 

 shaped skeleton. The polyps die and new 

 generations secrete new calcareous cups 

 upon the old ones; only the surface of the 

 coral mass is alive. 



CLASSIFICATION OF 

 THE COELENTERATA 



(For reference purposes only) 



This phylum includes polyps, jellyfishes, sea 

 anemones, and corals. All have a body wall 

 consisting of two layers of cells, between which 

 is a jell}like substance, the mesoglea, which 

 may or may not contain cells. Within the body 

 is a single gastrovascular cavity. The epidermis 

 is derived from ectoderm, and the gastrodermis 

 from endoderm. They are called acoelomates 

 because they do not possess a second body 

 cavity, the coelom. All coelenteratcs are pro- 

 \ided with nematocysts. 



About 10,000 species of coelcntcrates have 

 been described. They may be grouped into 

 three classes and two subclasses as follows: 



Class I. Hydrozoa. Hydroid polyps and medu- 

 sae with a velum; mesoglea noncel- 

 lular; solitary or colonial; mostly 

 marine. Exs. Hydra, Obelia, Gonione- 

 miis, and Physcilia (p. 121). 



Class 2. Scyphozoa. True medusae. No dis- 

 tinct velum; usually 8 notches in the 

 margin of the umbrella; mesoglea 

 cellular; polyp stage absent or re- 

 duced. Ex. Aurellia. 



