CORALS AND CORAL REEFS VAUGHAJST. 



191 



faces are covered with small short processes, termed cilia, which un- 

 der certain conditions beat so as to move the mucus and whatever 

 may be embedded in it toward the mouth, while under other condi- 

 tions they beat so as to move things away from the mouth. 



Sea anemones and corals are alike in the characters so far con- 

 sidered. They differ in that sea anemones have only soft tissues, 

 while the lower surface of corals secretes a skeleton, called the coral- 

 lum, composed mostly of carbonate of lime. Coral larvae, called 

 planulae, are small, pear-shaped or cylindrical objects, about half 

 a millimeter in diameter and about a millimeter long, and their 

 outer surface is covered with cilia by means of which they can move 

 rapidly. After a time, ranging from a day or two to two or three 

 weeks, the larval corals settle and attach themselves to some object. 



ABC D E 



Fig. 1. — Larvae of the coral. Favia fragum (Esper), mdch enlarged. 

 After Dueuden. The various forms of the larvae immediately after 

 extrusion are represented. a is viewed as a transparent object j b to 

 e are represented as seen by reflected light. the extrusion of 

 cell debris through the oral aperture of a is shown. 



At first a flat basal plate is secreted by the bottom end, and on this 

 are laid down radial plates that grow upward within or between the 

 mesenterial folds. Above each of the radiately arranged plates, known 

 as septa, there is a tentacle. At their outer ends the septa are joined 

 together by a wall, differing in character according to the kind of 

 coral, and at the inner ends of the septa there is usually, but not 

 always, a central columella, which likewise differs in character ac- 

 cording to the kind of coral. In the spaces between the septa pecul- 

 iar structures that are of much value in classifying corals* may 

 develop. 



Some corals remain simple, that is solitary, throughout their lives 

 (some of these are shown on pis. 3 and 12 to 14) ; while others 

 multiply asexually and form colonies. There are two kinds or 

 methods of asexual reproduction recognized by students of these 

 organisms. One of these methods, termed budding or gemmation, 



