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Each gill terminates within a pair of triangular‐shaped palps that are situated on either side of the mouth (Figure 2.6) and extend posteriorly about one‐third of the length of the mantle cavity (Morton 1992). The inner surface of each palp faces the gill and is folded into numerous ridges and grooves that carry a complicated series of ciliary tracts. The outer surfaces of the palps are smooth, and between the inner and outer surfaces there is muscular connective tissue (see Figures 4.15 and 4.16).

The main function of the labial palps is to continually remove material from the food tracts on the gills in order to prevent gill saturation. In dense suspensions, sorting and rejection tracts on the palps channel most of the filtered material away from the mouth and deposit it as pseudofaeces so that the animal can continue to filter and ingest at an optimum rate. The pseudofaeces is carried along rejectory tracts on the mantle to the inhalant opening and periodically forcefully ejected through it. When the ingestive capacity is not exceeded, particles from the gill move along acceptance tracts on the labial palps toward the mouth (see Chapter 4).