Pterobranchs are of particular interest to evolutionary biologists because as members of the phylum Hemichordata, they share characteristics with vertebrate animals and other chordates. The focus of this study is an examination of the development, structure, and function of the feeding arms in several species of pterobranchs collected from depths greater than 500 m from waters surrounding Antarctica. Pterobranch zooids in the genus Cephalodiscus feed using a crown of arms held over the dorsal surface of the body to filter particles from the water. Larvae released from adult tubes are ciliated, but lack feeding arms and are thought to derive energy from internal yolk stores. However, we have observed larvae of at least one species respond to the presence of particulate food, suggesting that these developmental forms may feed without the aid of adult structures. The feeding arms develop on the dorsal side of the animal, often beginning with a pair near the central axis followed by pairs of arms to the left and right. Each arm develops from a trilobed bud. The adult feeding apparatus consists of up to twelve arms held in a sphere on the dorsal side of the animal. Each arm has multiple tentacles, which are paired along the length of the arm until the apical-most tip. Scanning electron microscopy reveals that a single tentacle has two tracts of cilia along its outer face which may beat to draw water across the tentacular net or capture food particles. Food particles, including bacteria and single-celled algae, may then be conveyed down the tentacle to a deep, thickly ciliated groove on the outer face of the arm central to the paired tentacles, and eventually to the mouth. Scanning electron and light microscopy have elucidated the structures associated with feeding, as well as unusual refractive spheres at the tip of each arm in some species.



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