Examine the underside of the rock overhang and you should see greyish-white "threads" hanging down. These are colonies of hydrozoeans. Hydrozoeans are primative animals that belong to the same group as anemones and jelly fish (i.e., they are coelenterates). Each colony consists of hundreds of polyps. The feeding polyp has a circle of tentacles around the mouth. Cells on these tentacles help capture prey either by stinging and paralysing it, or by entangling it in threads and sticky secretions. The prey (small crustaceans, worms, larvae, etc.) is passed to the mouth by the tentacles. The digested food is distributed throughout the colony through the hollow "stem" of the colony. In addition to the feeding polyps, some polyps of the colony are specialized for reproduction. The reproductive polyps develop buds, which when released form small free-living medusae (ie. small jelly fish). The medusae produce sperm and eggs and the fertilized eggs develop into free swimming larvae called planula. The planula eventually attach themselves to the substrate and develop into polyps which produce a new colony by budding.
Collect some of the threads and examine them under a dissecting scope. Note the zig-zag stems characteristic of this species of hydroid (i.e., Obelia) and the feeding and reproductive polyps.