SHALLOW-WATER FAUNA OF THE TROPICS. 59 



branching canals ; and as spaces are left for 

 these when the shell is formed, the dried Coral 

 is perforated by numerous tubular pores, and has 

 a soft, spongy texture which can be easily 

 crushed into a powder if trodden upon. 



In other Corals the canals of communication 

 between the Polyps are entirely at the surface, 

 and the shell that is formed is much harder and 

 imperforate. In others again colonies are not 

 formed, but each individual grows to a consider- 

 able size and remains independent of its fellows 

 all its life. 



These, then, are three of the more important 

 varieties of Corals found on the reef, the Perfor- 

 ate, the Imperforate, and the Solitary Corals ; 

 but it must be remembered that all the Corals 

 of the reef cannot be included in these three 

 groups. The varieties are much more numerous 

 and in many cases much more complicated and 

 difficult to understand. 



Although the Coral-reefs of the Tropical world 

 have a general resemblance to one another, the 

 differences in detail are so great that it is impos- 

 sible to describe any one as typical. In sailing 

 over the edge of a reef near the coast on a calm 

 day, when the water is so clear that the bottom 

 can be seen at a depth of 8 or 9 feet, the reef 

 may, in some regions, be observed to change in 

 character every few minutes. In one spot there 

 may be clumps of living Corals surrounded by 

 beds of fine white sand; in another there will 

 be great stretches of branching Madrepores; in 

 another Madrepores, Mushroom-corals, the Im- 

 perforate Brain-corals, Sponges, and many other 



