158 THE SEAS 



of fine apertures each consisting of a central opening sur- 

 rounded by a ring of from five to seven smaller ones. 

 The former is occupied in life by a relatively stout, stumpy 

 polyp, which has a mouth and a digestive cavity, while 

 through the smaller ones project long, slender structures, 

 each with a series of little tentacles branching from the 

 main axis. These are all without mouths, but have 

 batteries of the stinging cells with which such animals 

 paralyse the minute animals on which they feed. These 

 stinging cells are so powerful that they can penetrate human 

 skin causing a painful nettle-rash, so that Millepora is known 

 as the Stinging Coral. Apparently the two types of polyps 

 work in conjunction, the one kind capturing the prey and 

 then handing it over to the large central polyp, which 

 swallows and digests it. As a result of continued branching, 

 great masses of Millepora are formed while fresh colonies are 

 begun by the attachment of the little embryos, which 

 develop from eggs produced, not directly by the Millepora, 

 but by very tiny swimming jellyfish, or medusae, produced 

 by the coral. 



Some of the " false," or Alcyonarian, corals form im- 

 portant constituents of coral reefs. In this group is 

 included the red coral of commerce which, as we shall discuss 

 in more detail in Chapter XVI, is never found in coral reefs, 

 being an inhabitant of temperate seas, and also the sea fans 

 and the dead-men's-fingers of our own coasts. But other 

 corals of this type are important constituents of coral reefs, 

 one of which, as shown in Plate 59, having a remarkable 

 type of growth, as a result of which the skeleton takes the 

 form of a series of little tubes running parallel to one anotlier 

 and united by little platforms, an arrangement which has led 

 to its being known to science as Tubipora inusica and to the 

 world at large as the Organ-pipe coral. The skeleton has 

 a deep red colour, but this is obscured in life by the emerald 



