466 EVOLUTION OF ANIMALS Part V 



hidden against the rock surfaces and seaweeds. Corals do not flourish in water 

 below 66° F. or in the deep sea. The white coral Astrangia, wiiich lives on the 

 Atlantic coast as far north as Massachusetts, grows only in small colonies, 

 never in the lush growths of the corals of tropical waters (Fig. 23.1 ). Except 

 for corals, coelenterates of one kind or another are at home from the far north 

 to the equator; the giant or pink jellyfish, Cyanea capillata, of the Atlantic 

 is common about Greenland. 



Food. All coelenterates are carnivores. Drifting jellyfishes are surrounded 

 by protozoans, entomostracans and numberless young larvae which they con- 

 stantly consume. To the sessile hydroids, sea anemones and corals, the tides 

 daily bring fresh supplies swept from the bottom and deeper waters off shore. 



Appearance and Size. Coelenterates occur in great variety. Branching colo- 

 nies of little hydroids are attached like plants upon the seaweeds. Jellyfishes 

 are bell- or umbrella-shaped and there are sea anemones more delicate and 

 colorful than their namesakes. The common names of corals are descriptive 

 of their forms — sea pen, sea fan, organ pipe, staghorn, brain, and mushroom. 

 Some jellyfishes are as transparent and colorless as crystal; others are trans- 

 lucent brown, deep red, yellow, lavender, or milky white. Colonies of the 

 hydroid, Tubularia crocea, common on the Atlantic coast, are rose-pink; 

 polyps of the organ pipe coral, Tubipora, have bright green tentacles and the 

 limy pipes of their skeletons are red. The Portuguese man-of-war floats on the 

 sea like a great opal, one of the most beautiful of marine animals. 



Hydroid polyps are usually very small, often microscopic, but colonies of 

 them extend over bands of seaweed for 50 yards or more. The diameter of 

 jellyfishes ranges from an inch, to 8 feet in the great pink Cyanea. Likewise, 

 sea anemones range from little ones with oral disks half an inch wide to giants 

 with a five-foot span. Although individual coral polyps are minute, the count- 

 less numbers of them in the colonies have built thousands of miles of coral 

 reefs and islands. 



Characteristics. Coelenterates are radially symmetrical and without head or 

 segmentation. The body is composed of two layers of cells, the external epi- 

 dermis or ectoderm and inner gastrodermis or endoderm, with a middle layer 

 of jellied mesoglea between them. Unique stinging cells containing the nemato- 

 cysts occur in one or both layers. The mouth, surrounded by soft tentacles, 

 opens into a saclike digestive cavity, the enteron, that may be branched or 

 divided by partial partitions and has no other opening. The skeleton is limy, 

 horny, or absent. There are no blood, respiratory, or excretory organs. A net- 

 work of nerve cells conducts messages through the body wall. Reproduction is 

 commonly by alternation of generations, with asexual budding from attached 

 polyps (hydralike) and with sexual reproduction by sex cells in the free- 

 swimming medusa (jellyfish) stage. 



Classes of Coelenterates. Hydrozoa. These are the little hydroids that grow 



