When they are ripe, the spermatozoa of one polyp are 

 released through the mouth and are carried by water 

 currents until they eventually reach another polyp. 

 Here they are drawn into the mouth by the water cur- 

 rents which are set up to bring food into the coral 

 animal. In this manner they reach the inside of another 

 polyp and fertilize the eggs while they are still attached 

 to the mesentery. 



After a short period of development the eggs are 

 set free and leave through the mouth of the parent. At 

 this stage they are in the form of small pear shaped 

 bodies about the size of a pinhead. Small rapidly beat- 

 ing hairs or cilia clothe the surface of the coral larvae 

 and give them a limited swimming ability. The stage 

 at which the larva is drifting or swimming feebly in the 

 water may last from one to three weeks. During this 

 time it may be carried considerable distances by ocean 

 currents and wind drifts, thus accounting for the pres- 

 ence of living corals on such isolated reefs as those of 

 Bermuda and many of the lonely Pacific islands. Within 

 a week or so, if the larva reaches a suitably hard surface 

 it attaches by means of a cementing substance formed at 

 one end and grows into a small tube. The upper end 

 is indented to form a fleshy cup which later becomes 

 the mouth. Tentacles grow out from the rim of the 

 upper surface and the skin immediately begins to form 

 the stony skeleton of the first polyp of a new colony. 



The tentacles are withdrawn during the daytime, but 

 at night they are expanded for the purpose of catching 

 food. This consists of small animals, principally Crusta- 

 cea, although some of the larger polyps, such as !Mani- 

 cina areolata have been observed to eat very small fish. 



Each tentacle possesses large numbers of stinging cells 



46 



