Three-section tubeworms

Three-section tubeworms (Family Chaetopteridae) have fragile bodies with three highly modified regions or sections: anterior (responsible for tube secretion and having tube-cutting bristles), middle (with surface of back specialized for feeding) and posterior (designed for gamete development). These unique suspension feeders strain seawater through mucus bags—a method that is very rare among invertebrates. For example, in the U-shaped parchment tubeworm (AN81), large, wing-like notopodia (dorsal branches of appendages) in the middle of the body secrete one large mucus bag within which suspended particles in the seawater are trapped. A cupule (small, cup-like structure) holds the tail end of the bag and, at short intervals, rolls the filled mucus bag up into a food bolus (ball). Posterior to the cupule, large fan-like notopodia that act like suction pumps or pistons move the water into and through the parchment tube. This action forces the water to flow through the mucus bag. Periodically the movement of water is reversed to assist the cilia (hairs) in the dorsal groove to transport the bolus forward to the mouth. For a U-shaped parchment tubeworm, the entire procedure of bag secretion and ball production takes approximately 18 minutes to complete. For the jointed three-section tubeworm (AN78), the process takes anywhere from 30 seconds to two minutes.

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