CH. Yi MABINE FAUNA OF TALIS SE SHOBES 129 



the surface, and several inches deep. On the outer edge 

 of the reef, where they are always covered with water, I 

 have frequently found them twice or three times as large 

 as this. The colour of the polypes is usually uniformly 

 deep brown, but many specimens may be found in which 

 the tentacles are tipped and speckled with a delicate emerald- 

 green. MTien the masses are covered with water every 

 polype is fully expanded, and they contract only very 

 gradually and slowly when touched or moved. Sometimes 

 a lump of Tubipora is found half in the water and half out 

 of it, and then may be seen every stage of contraction, 

 from the polypes that are tightly contracted and completely 

 withdrawn into the red tubes of the skeleton to those which 

 are stUl only half in and half out of it. It is very probable 

 that this power of complete retraction into solid calcareous 

 . tubes enables the organ-pipe coral to live in places which 

 are at times left partially dry at low water. Most of the 

 Zoantharian corals would in all probability suffer, and 

 perhaps die immediately under similar ckcumstances. 

 Before leaving the organ-pipe coral I must say a word or 

 two about the species of this genus. By collecting a large 

 number of specimens and making careful notes of the 

 colours of the polypes, I hoped to be able to arrive at some 

 trustworthy basis for the separation of species. The more 

 specimens I collected, the more surely was I driven to the 

 conclusion that there is only one true species to be found 

 on the reefs of Talisse. The length and diameter of the 

 tubes and the appearance of the horizontal platforms are 

 the features which are usually used by naturahsts for the 

 separation of species of Tubipora. These, however, are 

 not, I am persuaded, sufficiently trustworthy for the pur- 

 pose. My investigations proved conclusively that the 

 length and diameter of the tubes depend entirely upon the 

 position of the coral on the reefs. If it is found on the 



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