526 A NATURALIST OX THE " CHALLENGER." 



species. The tips of the lobes of the living coral are of a bright 

 gamboge-yellow colour, which shades off into a yellowish-brown 

 on either side of the lobes. Mr. Murray succeeded in getting 

 the polyps of the coral to expand under the microscope, and 

 handed them over to me for examination. I found them, as 

 Agassiz had discovered long before, to be Hydroids allied to the 

 Medusae and not to the Actinozoa and Sea Anemones, like the 

 majority of modern stony corals ; I studied the structure of 

 the coral minutely.* 



The hard part of the coral or calcareous skeleton is finely 

 porous throughout, being excavated by a complex reticulation of 

 fine and tortuous canals which are in the freest possible com- 

 munication with one another. Within this porous mass at its 

 surface are excavated cylindrical holes or pores of two sizes. 



The canal spaces in the skeleton are, when the coral is living, 

 filled by a network of living tissue made up of a meshwork of 

 branching and communicating tubes, which form a canal system, 

 by means of which a free circulation can pass from one part of 

 the coral to another. 



Two kinds of Polyps inhabit the pores described as existing 

 on the surface of the coral. The larger pores are occupied by 

 short stout cylindrical polyps which have each four tentacles 

 and a mouth and stomach, and which are hence termed " Gas- 

 trozooids," whilst their pores are termed " Gastropores." The 

 smaller pores shelter each a very different kind of polyp, which 

 has a long and slender sinuous body provided with numerous 

 tentacles, and devoid of any mouth or stomach ; this latter form 

 of polyp, because its function is merely to catch food, is called a 

 " Dactylozooid" and its pore a " Dactylopore" 



The Dactylozooids catch food for the colony and deliver it to 

 the Gastrozooids, which alone are able to swallow and digest it. 

 All the polyps of the colony are in communication at their 

 bases with the canal system already described, and by means 

 of these canals the nutritive fluids derived by the gastrozooids 

 from the food are distributed to the entire colony and nourish 



* For further details, see H. N. Moseley, " On the Structure of a species 

 of Millepora occurring at Tahiti." Phil. Trans. Roy. Soc, Vol. 167, 1877, 

 p. 117. 



