140 MR. A. E. FINCKH. 



Similar patches of the Porites limosa are seen a little to the south of the 

 obelisk. These are not so extensive but, being in deeper water and therefore never 

 uncovered, they assume a greater thickness. Their upward growth is not interfered 

 with by the level of the tides, so that their uppermost portions are not' in the same 

 plane ; nor does the Lithothamnion as yet encroach upon them. 



In other localities the Porites on the platforms are not seen to form continuous 

 masses, but their coralla invariably assume the pancake shape, and have always a 

 depression near the centre, varying with the size of the colony. This depression 

 contains living Lithothamnion, which forms the centre from which the alga will 

 eventually extend over the entire surface of the Porites, converting it thus into the 

 reef rock of the platform. Porites of this definite shape and these habits are met with 

 very frequently, but the difference in shape and mode of expansion of tliese specimens 

 as compared with those growing in deeper water points to the fact that their presence 

 in these former situations is accidental, while their growth is forced and unnatural. 



Madrepora loripes, Brook, and the different varieties of jPoctYZojooj-cr, especially the 

 species P. ccBspitosa, do not merit further description than that already given. 



(4) The last reef-forming organisms, the Foraminifera, were not studied by me.* 



(5) Reef- Destroying Organisms. 



In the case of the living corals no organisms were observed to burrow In, or to feed 

 on, them, so as to cause their destruction, or otherwise interfere with their growth. 

 The branching corals are particularly free from any such attacks. The massive corals, 

 such as Porites, and of the Hydrocorallinae, the stout Millepora complanata, almost 

 invariably harbour molluscs, both bivalve and of the tunnel-forming type, together 

 with commensal cirripedes. But all of these, more or less, remain stationary, and do 

 not interfere seriously ^^^th the welfare of the coral, though they, of course, enlarge 

 the tube or cavity in which they live at its expense. 



The Lithothamnion rock, however, especially on the eastern rim, is interfered with 

 by burrowing organisms to a great extent, so that these must be a considerable factor 

 in the economy of the reef. The chief of these destroyers is a Gephyrean and an 

 Aspidosiphon which is from 2 to 3 inches in length, but very thin, and possesses a 

 long retractile proboscis by which it is seen at low tide to feed off the Lithothamnion 

 covered rock immediately surrounding its abode. By these the eastern ocean 

 platform near the edge, indeed the entire iMhothamnion zone, is literally riddled. 

 A small piece of the reef rock a few Inches in diameter when broken off will show 

 as many as 10 or 15 of these animals, and, before it can be quite severed from the 

 rest of the rock, the bodies of these have either to be dragged out of their habitations 

 or to be separated. 



* See Mr. F. Chapman's description, ' Linnean Soc. Journal ' (Zoology), vol. xxviii. (1900-03), pp. 1, 161. 



