114 CORALS 



contraction the tentacles are first folded inwards over the 

 mouth, and then the whole crown of tentacles, mouth, and 

 stomodaeum are drawn downwards into the tube, and this 

 is followed by the infolding of the body wall from above until 

 the limit of the red tube is reached. When the contraction 

 is complete the mouth of the tube is stoppered by the con- 

 tracted polyp, and thus the exit of the water from the body 

 cavity is prevented and the coral is able to retain its vitality, 

 even if the coral, by the fall of the tide, is left for a few hours 

 exposed to the tropical sun. The tubes are built up by the 

 growth and fusion of a large number of spicules of calcium 

 carbonate in the substance of the body wall. In the upper 

 part of the contractile part of the body wall the spicules 

 are small and scattered, in the lower part they are much 

 larger, and in the region of the junction of hard and soft 

 parts they have become so large that they are articulated 

 together to form a firm skeletal wall. 



The firm coral substance or " corallum " of Tubipora is 

 constructed, therefore, in the same way as it is in Corallium, 

 by the fusion or, to be more correct, the jamming together 

 of Alcyonarian spicules. But whereas in Corallium the 

 substance thus formed is quite compact, in Tubipora a 

 number of spaces or pores are always left in the substance, 

 by which the living tissues can maintain a connexion 

 between the endoderm lining the inside of the tube and the 

 ectoderm covering the outside. The Organ-pipe coral is 

 therefore a perforate coral, and, like all perforate corals, its 

 substance is brittle, and is rapidly broken up and disin- 

 tegrated when exposed for any length of time to the action 

 of the surf. It is also a tabulate coral, but the tabulae 

 are very variable in form and frequently so different in 

 character from the tabulae of Millepora, Heliopora, and 

 many other corals that the name " tabula " does not seem 

 to be strictly applicable. 



In some tubes there may be found a flat plate of coral 

 substance, dividing the cavity of the tube transversely on 

 the level of a platform. Such a plate is obviously a tabula 

 of the ordinary type. In other places the tabula is cup- 

 shaped, and more frequently it is drawn out into a fine point 



