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The food tract is continuous in front with the right side of the foot, and the lateral tracts of the foot on either side converge immediately below the proboscis. At this point the paired pedal tentacles arise close to the midline, and between them is situated the opening of the duct of the pedal mucus gland. In front of the gland opening is located a small triangular area of glandular epithelium, representing the original sole, or plantar surface of the foot (Fig. 3, G.FT.).
Text Fig. 3—Serpulorbis zelandicus. Diagrams showing the structure of the foot on emergence of the embryo from the capsule (A) and its modifications in the adult (B). F.TR, ciliated and glandular food tract; GL.FT, glandular sole or plantar surface of foot; OP, operculum; PD.G, opening of pedal gland; PD.T, pedal tentacle.
An understanding of the modifications that have taken place in the foot may be gained from Text-figure 3 and from Figs. 20–22 of developing embryos. In the capsule veliger (Fig. 21) the foot is triangular, with a transverse opercular rudiment upon its posterior surface and a bifid tubercle at the anterior edge, giving rise to the pedal tentacles. In the embryo which has just emerged from the capsule (Fig. 22) the velum is lost, and the foot has the same relations as in a typical adult free-moving prosobranch. The plantar surface is wide, enabling the animal to creep about, and the operculum is very prominent, forming a large saucer-like structure (Fig. 22 op.) somewhat overlapping the edges of the foot. Its concave shape is strongly reminiscent of the adult operculum retained throughout life in “Vermetus” novae-hollandiae. The anterior edge of the foot is bluntly rounded, with strong cilia, and in front of the foot is a pair of well-developed pedal tentacles, rugose and finely ciliated. The
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sides of the foot are covered with fine cilia, and correspond to the lateral tracts of the foot in the adult. They carry waste particles backward where they are rejected along the edge of the operculum. In the adult Serpulorbis zelandicus the operculigerous disc of the foot is much enlarged and bereft of its operculum; it now forms the circular terminal disc closing the shell aperture. In Bivonia triqueter (Lacaze-Duthiers, op. cit.) a tiny vestige of the operculum remains in the centre of the disc; in Serpulorbis gigas, as in S. zelandicus, it is lost altogether. In the adult the sole is compressed to a very small size; its surface is provided with unicellular mucous glands, and its cilia beat towards the terminal disc, serving for the removal of waste particles.
The anterior margin of the sole now forms a transverse lip, somewhat overlapping the pedal gland opening; it corresponds to the lower lip as described by Yonge and Iles (1939) in S. gigas. The pedal tentacles (Fig. 3, pd.t.) on either side of the opening are long and tapering. They are covered with a thin cuticle, save for a narrow ciliated tract along the mesial edge, which may be infolded to form a deep groove. These tracts are continuous at the base with the opening of the pedal gland (pg.o.), carrying outwards a constant supply of mueus. Food particles admixed with mucus in the pallial cavity are carried forward to the opening of the pedal gland. Here a small bolus of mucus appears to be held between the vertical pedal tentacles, and rounded off before ingestion at the mouth. Probably an added secretion of mucus is received from the pedal gland. The bolus is nipped off from the surface of the foot by the sharp edges of the jaw plates, and raked into the buccal bulb by the sharp radular teeth.
The pedal gland (Fig. 1, pg.) is a large, yellowish-white mass, situated in the trunk cavity immediately behind the pharynx, ventrally and to the right of the anterior portion of the oesophagus. As described by Yonge and Iles in Serpulorbis gigas it is “heart-shaped in transverse section,” being incised ventrally by a wide duct that runs forward below the pharynx. It is much better developed than the rather narrow glandular strip in “Vermetus” novae-hollandiae, and though relatively shorter than in Serpulorbis gigas, it is equally stout, displacing the oesophagus to the left side (Fig. 4). The histology is illustrated in Figs. 5, 6. The gland is built up of a close-set mass of lobules formed of spherical or polygonal secreting cells, 5μ in diameter. The nuclei are large and round, and the cell contents coarsely granular, staining deep brown with Van Giesen's. Towards the ventral aspect appear a series of crowded ductules, each 7–8μ across, formed of cubical or flattened cells. Very long cilia occupy the whole lumen and lash the mucus secretion forwards and downwards towards the main longitudinal duct. The main duct is approximately 1/3–1/2 mm. in width; its ventral wall is composed of a series of longitudinal ridges, formed by differences in cell height. The median fold is most prominent and the summits are strongly ciliated, with a forward beat. The dorsal wall is penetrated from above by the smaller ductules and is composed of flattened, squamons, non-ciliated cells.
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The highly developed pedal gland is evidently not employed solely in connection with ciliary food collecting. In Serpulorbis gigas Boettger (1930) has shown that food is entrapped by means of long extruded mucus strings, formed by the pedal gland. Three or four strings are moved gently to and fro, and are periodically pulled in and ingested together with the planktonic organisms collected. In a further review of Serpulorbis gigas, Yonge and Iles (op. cit.) point out that the pallial cavity has entirely lost its ciliary feeding mechanism; the ctenidium is reduced, and weakly ciliated, the food tract little developed, and only a feeble water current is maintained through the pallial cavity. In Serpulorbis zelandicus, mucus threads are undoubtedly formed by the animal in its natural location, but were seldom able to be satisfactorily observed. They were sometimes seen at low tide on overturning a rock, when a thread two or three centimetres in length could be identified still attached to the opening of the pedal gland. The threads were always thin and delicate, though it would appear that when the animal is covered with water, and wave surge is reduced, mucus traps may be put out without interruption to form a supplementary means of food collection. The high degree of development of the pedal gland strongly supports this suggestion. “Vermetus” novae-hollandiae, which is typically a species of rough water, is stated never to form mucus strings: Serpulorbis gigas, which subsists entirely by mucus feeding, is a calm-water species. The pallial organs in Serpulorbis zelandicus are relatively much better developed than in gigas, corresponding with the retention of ciliary feeding. Zelandicus is clearly a transitional form between the V. novae-hollandiae and S. gigas types, and makes it easy to see how the two extreme groups of Yonge and Iles may be related. All attempts to observe the formation of mucus traps in zelandicus in the aquarium were unsuccessful. A large bolus of mucus was generally extruded indecisively over the rim of the operculigerous disc, but was never elaborated into strings. Yonge (personal conversation) states a similar difficulty was found in inducing gigas to feed under laboratory conditions.