-10- major concentrations are to be found in the leaves, bark, seeds, corms or nuts. Since these are the most used portions of the plant for animal foods, the plants pass the radioactive materials on to the animals where they are incorporated into the animal tissue, only to be released in the normal biological cycle and passed back to the land for reabsorption. The invertebrates, or animals without backbones, make great bulk of the animal life of an atoll. up the The role of these animals in the cycling of radioactive materials in an atollis as varied as the invertebrate forms. Sea cucumbers have been compared with earth worms in their ceaseless turning of the gravel and sand as they obtain their nutriment from bacteria and algae. Corals and clams remove microorganisms and particulate matter from the water and also host to the unicellular algae, Zooxanthellae, which are found tissues. are in their The Zooxanthellae may be thought of as a vast reservoir of trapped plankton. understood but Their relationship to their host is not completely it is probable that they play an important part in the removal of phosphate wastes. Corals and clams are eroded by algae and sponges, which bore holes in the skeleton or shell, thus contri - buting to a return of carbonates to the water. Crabs, sipunculid worms and others also attack the skeleton of the corals. land crabs contribute to the deposition of radioisotopes Some of the from the sea onto the islands by dragging fish and algae ashore when feeding.