It's the worms' turn

摘要

Worms are the earth's custodians. Many ingest and detoxify the ocean's noxious biotic debris, and the Sabellid worms are among the most numerous of these stewards. Sabellids are polychaete worms found in all the oceans, and are surprisingly common. Coral reefs are rife with them. Almost all Sabellids are filter feeders and, as such, they remove organic refuse or plankton that floats near the bottom or swirls on the currents. The head of sabellids has been transformed into a feathery "crown" comprising radioles that project from a protective tube into the water. This crown filters out the necessary food, but this requires time, meaning long exposure to potential predators. The photograph shows a sabellid (Megalomma sp) with two compound eyes at the end of two of the crown's radioles; these aren't used for feeding but project the furthest away from the worm giving it a wide view. Each sabellid "eye" consists of 40-60 ommatidia, but these "little eyes" are different, anatomically, from those seen in insects or spiders. The individual ommatidium is a tapered pigmented tube resembling an ice cream cone formed from a single cell with a crystalline core as the "ice cream." The apex of the cone contains a single photoreceptor that is connected to an axon. The. cone-shaped pigment cell serves to isolate the receptor portion from stray photons and to provide directionality.