The chordoid larva of Symbion pandora (Cycliophora) is a modified trochophore

Developmental and free-living stages of the chordoid larva of the cycliophoran species, Symbion pandora Funch and Kristensen 1995, were studied using light and electron microscopy. In the free-living stage of the larva, about 200 μm long, four ciliated areas are found: two anterior bands, a ventral ciliated field, and a posterior unit on the ventral side of the foot. The nervous system consists of a dorsal brain and a pair of ventral longitudinal nerves. A gut is absent. A pair of protonephridia, each with a single multiciliated terminal cell and at least one duct cell, is present. Nephridiopores are not localized. A pair of corsal ciliated organs is posterior to the brain. The homology between these and the apical organ of a trochophore larva is discussed. A distinctive longitudinal rod, the chordoid organ, consists of vacuolized cells with circular myofilaments. The organ is comparable to a similar structure in gastrotrichs. In the discussion of the phylogenetic position of Cycliophora among protostomians, important morphological observations that are described in the present study indicate that, despite some dissimilarities, the chordoid larva is a modified trochophore. © 1996 Wiley-Liss, Inc.