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In their relationship to the general ecology of the reefs, the Mollusca as a group of animals play a highly significant role. Because of the nature of their shells, mollusc remains may be found among the limestone debris of a reef dating back to its very earliest stages of evolution in the geological past, and may, therefore, be considered as having aided in its construction. Yet, as boring organisms, in both living and dead coral, certain species of molluscs rank among the most destructive agents to be found on the reefs. Mollusc eggs are laid in tens of millions, and the floating larval stages form a very important part of the zooplankton that sustains life in the waters over the reefs. In their vast numbers, as herbivores and carnivores, the molluscs are both prey and predator on the reefs.

Most molluscs obtain their oxygen directly from the seawater by means of gills in the mantle cavity. Molluscs with two valves, or shells, hinged together are referred to as bivalves. In bivalves the gills have become so developed that in many species they also act as the food-catching apparatus. Because most bivalves are filterfeeding animals, sieving minute organisms from the surrounding seawater, they have no differentiated head region and no radula (tonguelike organ).

It is in the method of feeding that the clams of the family Tridacnidae display their remarkable adaption to their environment. In the course of their evolution, the various members have become structurally modified to best enable them to develop in their particular habitat. These clams normally live with the animal lying on the hinge side of the shell, and the edges of the shell valves pointing upwards. The result of this mode of living is that the internal organs in their relationship to the mantle and shell differ from those of all other bivalve molluscs. The mantle tissue, by reason of the animal's way of life, is capable of considerable expansion, and is exposed to the direct rays of the sun to the greatest possible extent. This is undoubtedly associated with the most unusual feature of all displayed by these remarkable molluscs: Within this mantle tissue are millions of tiny zooxanthellae, closely resembling the symbiotic algae found in the corals and the alcyonarians. It has been definitely established that these zooxanthellae form a considerable part of the diet of the Tridacnas, and the modifications found in these molluscs indicate that they are not only specialized for harboring these minute algae, but that they also deliberately "farm" them. This must surely be one of the most fascinating examples of symbiosis to be found in nature. The whole ecology of the clams, bound exclusively to warm, shallow waters of tropical seas, appears linked to an increase in the efficiency of that symbiosis, which gives food and protection to the algae and very considerable additional nutrient from its symbionts to the clams.

3. An appropriate title for this passage would be:

(A) Filter Feeding in Bivalve Molluscs
(B) Plankton in Coral Reef Ecology
(C) Prey and Predator in the Tropical Seas
(D) Role of Molluscs in the Reef Ecosystem
(E) Molluscs and Coelenterates of the Coral Reef