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Summary.

The coral reefs off Ghardaqa are pictured and their conditions described in regard to growth and to decay. Attached corals living on the more seaward reefs are frequently broken off by strong waves. The effect of such on Acropora, Lobophyllia, and Porites is examined, and it is estimated that 60 per cent, of the broken-off corals were weakened by boring-sponges, 20 per cent, by boring molluscs, defects in growth form being responsible for the remainder.

The infection of corals by boring-algæ is studied. The algal zone is seen in a green coloration extending about 7 mm. below the surface-layer of polyps; in the deeper skeleton yellow concentric markings indicate its former position. Almost all dead coral is infected.

The boring animals are reviewed, the sponges, worms, and bivalve molluscs (Lithophaga) being especially considered.

The undercutting of cliffs of elevated coral rock is important. Boring-algæ by softening the surface of their limestone proved to be the chief factor in their formation. Boring and rasping organisms are also destructive. Solution of the rock acts through the organisms, not directly.

Holothurians were examined in respect to their supposed action in grinding down sand in its passage through their bodies. They were studied both on the reefs and in tanks. In feeding they were often found to select the larger grains from any sand surface on which they were living. The sand contents of the head and tail portions of the gut of 64 specimens were sieved in comparison with the sand on which they were living. The opinion is expressed that any grinding or solvent action in the gut is small.