224 THE INFLUENCE OF INANIMATE SURUODNDINGS. 



species; on the more massive ones, either funnel-shaped or 

 cylindrical pits are formed. In tha course of my travels I have 

 made numerous other observations as to the similar effects of 

 currents on individual coral animals and on whole blocks of coial. 

 It will suffice here to select two particularly instructive cases. 



One of these bears upon the growth of certain massive 

 species of coral, among which we may especially consider the 

 species of Porites, so common in reefs, and a few of the 

 AstrceidcB. The smallest, and so the youngest, colonies had, as 

 a rule, a regular convex surface ; but only, of course, when they 

 were healthy and not eaten away by other animals or by 

 plants. In them the polyps situated at the summit were as 

 well developed as those at the margin of the mass, and they 

 were never left dry even by the lowest ebb-tide. Larger blocks 

 of the same species, which seemed to be more often exposed to 

 the effects of the air, were flat ; the polyps at the summit looked 

 feeble, and many of them were dead ; indeed, small stones and 

 sand were often lying on the centre of the surface. In still 

 larger masses, of which the upper surface was commonly laid 

 bare by even ordinary ebb-tides, nearly all the polyps at the top 

 were dead, and often entirely covered with sand, NuUipora, and 

 other Algse. The summit of the largest stocks, finally, was 

 concave, with a raised border, from a few lines to an inch 

 higher than the central portion. These old stocks exhibited 

 some important peculiarities. It is well known that even in 

 corals of a massive and solid type there are often slight furrows 

 between the separate cups, and in many stocks of Porites of 

 moderate size these are enlarged to trenches of various breadth. 

 In the largest, again, they have become naiTOw but often veiy 

 deep channels which traverse the concave surface and even the 

 raised margin in various directions. 



We can, as it seems to me, without any forcing, avail our- 

 selves of the conditions here described to construct a theory of 

 the process of growth of a knoll of Porites. So long as 

 the young colonies are completely under water even at the 

 lowest tides, the separate polyps grow out in every direction, 

 giving the coral an equal convex surface ; a section through 

 it gives an outline like that shown in the subjoined wood- 



