Organization of pharyngeal hard parts and musculature in Gnathostomula armata (Gnathostomulida: Gnathostomulidae)

The pharynx of Gnathostomula armata, like that of other members of the phylum Gnathostomulida, consists of a set of jaws, a basal plate, and a muscular bulb that encloses these cuticularized hard parts. Field-emission scanning electron microscopy (SEM) provides additional information about the hard parts and shows that the dentition of the jaws is arranged in three rows: 7–10 teeth in a dorsal row, 16–20 teeth in a medial row, and 20 teeth in a ventral row, a pattern different from that reported from light microscopy (LM). SEM also shows that the dentition of the basal plate is more like that of other Gnathostomula species than was previously discerned. Confocal laser scanning microscopy shows the musculature of the pharyngeal bulb to comprise diductors that open and tilt the jaws, looplike abductors that retract them as they snap shut by recoil, and a pair of inclinators and pair of levators that also participate in tilting the jaws back and forth. A constrictor running ventral to and behind the jaws may work to protrude them. Two arc-shaped muscles attached to the basal plate pull it forward and tilt it down to scrape food from the substratum so that it can be grabbed by the jaws. Paired retractor muscles pull the basal plate back into the mouth.