old, more likely ninety, but they had undergone no 

 obvious changes during all the years of observation. 



THE TRUE OR STONY CORALS 



The graceful sprig of white coral on the mantel- 

 piece, rudely broken from its firm attachment on 

 some coral reef, is little more than a brittle limestone 

 cast of coralline symmetry. In life it was veiled with 

 delicate flesh of pink, heliotrope, purple, red, yellow, 

 green, or golden-brown hue; and it held blossom-like 

 polyps secure in its sheltering craters. Though many 

 tons of coral skeletons are every year distributed all 

 over the world to ornament homes far from the trop- 

 ics, the great economic importance of coral polyps 

 lies in the serious hazards to navigation erected by 

 their limestone-secreting habits. So rapid is reef 

 growth in some parts of the South Seas that navi- 

 gation charts more than twenty years old are said to 

 be useless. Much modern research on living coral 

 reefs is contributing toward a more successful ap- 

 proach to drilling for oil in fossil reefs left from ear- 

 lier geologic periods when the extent of warm seas 

 on our watery planet was far greater than it is in 

 our time. 



There are some twenty-five hundred species of 

 true or stony corals (technically called scleractinian 

 or niadreporarian corals), and all have similar pol- 

 yps that look like tiny and delicate anemones sitting 

 in limestone cups. The polyps may be widely spaced, 

 each occupying a separate cup; or the cups may be 

 so close together as to have common walls; or the 

 polyps may be joined together in rows and occupy 

 grooves in a rounded skeletal mass. In the "brain 

 corals" so common on coral reefs, sinuous skeletal 

 grooves are fringed on each side by a continuous 

 row of tentacles and have along their bottom a row 

 of spaced mouths. 



Relatively few corals are solitary, and these oc- 

 cupy isolated little cups or disklike skeletons several 

 inches across. All the rest are colonial and join their 

 small but numerous forces to secrete large coral tene- 

 ments. The rounded boulder-like corals are hardier 

 and predominate at the surf-beaten seaward face of 

 most reefs. The branching antler-like corals (Plate 

 3 1 ) of shallow waters are more typical of the pro- 

 tected rear areas of a reef. Sometimes the same spe- 

 cies of coral grows rounded or softly lobed in exposed 

 situations and intricately branching farther back on 

 the reef. Deep-water coral colonies have a treelike 

 aspect, with narrow branches well suited to shed sed- 

 iments that fall from above. 



In the daytime coral polyps remain more or less 

 contracted, then expand and feed at night, when 

 plankton animals rise to the surface in greatest num- 

 bers. Corals with very small tentacles entangle mi- 

 nute crustaceans and other animals in strands of mu- 

 cus and waft them to the mouth by beating cilia. The 



larger polyps with long tentacles grasp small prey, 

 sometimes even tiny fishes, and drop the food onto 

 the mouth or push it in ( Plate 26 ) . 



The skeletons of stony corals are not laid down 

 within the living substance, as in the alcyonarian 

 corals described earlier, but are secreted by the outer 

 layer of cells and lie completely outside the coral 

 animals. Each polyp secretes about itself a limy cup 

 filled with radiating ridges that alternate with the 

 internal partitions. As the ridges grow by steady ac- 

 cretion, they push up the underside of the body into 

 folds that conform to the hard ridges. Except for 

 some of the orange-red or red solitary and deep- 

 water forms, corals have no pigments in the skeleton 

 itself, so that dried reef corals are various shades of 

 ofT-white until they are bleached white by the sun or 

 by those who prepare pieces of decorative coral for 

 sale. Many reef-coral "heads" look red or green 

 when broken open, but only because the old layers 

 of porous colonial skeleton are thoroughly permeated 

 by colored algae. 



Corals live firmly cemented to the bottom, but 

 some of the solitary forms, though attached when 

 young, are freed later in life to shift about on sandy 

 bottoms or to become imbedded in mud by a pointed 

 base. The mushroom coral. Fungia, found mostly on 

 tropical reefs, has a single large green or brownish 

 polyp that may be 5 inches across or more. The fully 

 extended tentacles stretch 2 or 3 inches beyond the 

 disk. The young mushroom coral expands at the 

 mouth end into a disk which is eventually set free. 

 The original stalk then repeatedly produces and 

 sheds disks until a number lie scattered about, and 

 still growing, upon the bottom. The beautiful convex 

 disk with its many large radiating ridges looks like 

 the underside of the cap of a gilled mushroom; it is 

 familiar to collectors of shells and corals. A free- 

 living coral, Heteropsammia, provides shelter for a 

 sipunculid worm; when the coral topples over, the 

 worm sets it upright. 



The subtle or gorgeous colorings of living corals, 

 which make coral reefs as exquisitely beautiful as 

 any flower garden, are provided in part by the 

 golden-brown plantlike cells that live within colorless 

 polyps, as well as by the many pastel tints lent by 

 pigments in the flesh. The gayest contrasts often 

 come from the other animals that throng all coral 

 reefs, either attaching firmly to maintain a foothold 

 on these biological oases in a vast, shifting ocean, or 

 moving about freely from one coral crevice to an- 

 other in the almost spongelike porosity of old coral 

 layers. Gaily colored fishes dart in and out of coral 

 thickets, and some of them browse on the coral to get 

 at the worms in coral cavities. Little coral-gall crabs 

 live within the branches of certain corals, the young 

 female settling in the fork of a growing branch and 

 becoming imprisoned as coral growth continues. In 



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