Elongate dorsal appendages adorn pelagic larvae of many fishes from disparate taxa, among them larvae of the family Carapidae, wherein the singular, elongate appendage is termed a vexillum, The vexillum is a complex larval specialization of the dorsal fin and is characteristic of all carapid genera, It is motile, well vascularized, and innervated by a branch of a cranial nerve with no apparent spinal innervation. Histological studies of Echiodon dawsoni and Carapus bermudensis vexilla show a two layered epidermis with mucous cells, an arteriole and venuole, two myelinated peripheral nerve fascicles, and two collagenous central shafts. No taste buds, free neuromasts or organized chemoreceptors were found on the vexillum, including its fleshy tabs; collateral branches of the nerve fascicles may end as free-nerve endings. In situ observations of living pearlfish larvae revealed a remarkably long and ornamented vexillum that assumed variable postures. The data suggest that carapid vexilla serve multiple functions, including predator deception, sensation, and hydrodynamic effects
