The Structure and Adaptations of the New Zealand Vermetidae
Part III. Novastoa Lamellosa and Its Affinities
[Read before the Auckland Branch, May 24, 1950; received by Editor, May 25, 1950.]
The structure and biology of Serpulorbis, Stephopoma and Pyxipoma have been discussed by the present writer (1951, 1951a) and the family Vermetidae as traditionally recognised has been divided into two natural groups by the introduction of the family Siliquariidae, to accommodate the two last-named genera. The survey of the New Zealand vermetids may now be completed by a short account of Novastoa lamellosa. Hutton (1873) established his Siphonium lamellosum with a rather meagre description of Cook Strait shells, without a figure. Finlay (1928) gave a fuller description of the shell and operculum, but followed Suter in assuming specimens of lamellosum to be identical with Vermetus zelandicus Q. and G. Quoy and Gaimard's vermetid was accorded no original shell description, and can be determined only by the coloured figures of the animal in the authors' Atlas of Zoological Illustrations. These are, however, accurate and well drawn, and show the species to belong to Serpulorbis. Novastoa lamellosa was thus first validly described by Hutton. The present genus was established by Finlay (op. cit.) and the New Zealand species is the genotype.
The occurrence of living Novastoa lamellosa is discontinuous and very local. Cranwell and Moore (1938), in their survey of plant and animal littoral communities at the Poor Knights Islands, give an excellent account of the ecological relations of Novastoa, which is there one of the dominant sessile animals. Other living occurrences are from Mokohinau Is. (C. A. Fleming), Tiri Tiri, and Little Barrier Id. (A. G. Stevenson), while shells are frequently washed ashore on the eastern side of Coromandel Peninsula. The species is probably a littoral dominant in many of the offlying east coast islands. Finlay, recording it from the Chathams, states it is elsewhere purely Cookian. In February, 1950, living specimens were collected and examined alive on Poor Knights Islands with the kind co-operation of Mr. R. Morrison Cassie, Fisheries Biologist, New Zealand Marine Department, and Captain A. Duthie, Master of the Research Vessel Ikatere, to both of whom the writer is much indebted. His special thanks are due also to Dr. Myra Keen, of Stanford University, California, for so generously providing comparative materials of Petaloconchus, Spiroglyphus and Aletes.
Among the vermetids of which the animal has been examined in detail, Novastoa shows the closest resemblance to “Vermetus” novaehollandiae, as described by Yonge (1932). It is predominantly a ciliary feeder, collecting food particles by the ciliary currents of the ctenidial filaments within the pallial cavity. Like Serpulorbis zelandicus,
however, and perhaps a majority of vermetids, it appears to rely also, to a smaller extent, upon mucus trap feeding. Novastoa lamellosa differs from Serpulorbis zelandicus in its position in the littoral, occurring much higher on the shore, immediately below the sessile cirripede formation on Poor Knights either Elminius plicatis or Chamaesipho brunnea. Possessing a close-fitting operculum when the foot is withdrawn into the shell, the animal is much more tolerant than Serpulorbis of exposure between tides and, like barnacles, to considerable extremes of temperature. The lower margin of the Novastoa zone is approximately at low-water neap tide—it is thus ecologically equivalent to the zone of serpulid worms—an association which seems to be unrepresented at Poor Knights Islands; it is the dominant species in Cranwell and Moore's Novastoa-encrusting coralline association. Serpulorbis zelandicus by contrast apparently never reaches the status of a dominant sessile organism—the animal is much less protected, having no operculum and relying only on deep retreat into the shell tube; it occurs at much lower tide level, singly or in patches, under stones in clean water at extreme low spring tide mark, in the zone of emergent brown algae (Carpophyllum) and encrusting pink corallines, or well below low tide on laminarian holdfasts. The third encrusting vermetid represented in the Cookian province is Stephopoma roseum, which may become locally dominant under stones, and just below fringes of emergent Corallina. Its tidal position is slightly higher than that of Serpulorbis, and it is a good deal more tolerant of sediment. Where the two forms occur together—as in the Noises Group (Morton, 1951)—Stephopoma forms a continuous strip just above low spring-tide mark, and Serpulorbis is found in patches with pink Melobesia, immediately below.
The leading ecological features of Novastoa lamellosa are summarized by Cranwell and Moore: “Still water is antagonistic to this species, which seems to march parallel with the other communities all round the islands on steep or moderately inclined slopes, but is lacking or poorly developed in tide pools or on shelving rocks where the water is likely to lie.” A preference for disturbed water is shared by the Australian “Vermetus” novae-hollandiae (Yonge, 1932), which occurs as a ciliary feeder on exposed portions of the Barrier Reef. Novastoa, however, would not appear to tolerate positions of maximum exposure to surf—where—on Poor Knights—Chamaesipho extends right down to D'Urvillaea or Xiphophora and the vermetid zone is cut out.
The general structure of the head, foot and pallial cavity of Novastoa (Plate 8, Fig. 1) resembles that of “Vermetus” novae-hollandiae. When the animal is extended during feeding, the opercular disc is raised, the head and foot widely protruded, and the margin of the mantle widely everted to lie over the rim of the shell tube. The animal is handsomely and distinctively coloured. The head (cph), terminating in the short cleft proboscis, is jet black behind, encircled in front by a broad scarlet band, widest laterally—just behind the tentacles, with a lighter yellow patch below at the base of either tentacle. The cephalic tentacles and the lobes bordering the mouth at the sides are black. The upper and lower borders of the mouth and the entrance to the buccal cavity are orange. The plug-like foot is whitish behind, orange-brown dorsally along the ciliated tracts (a.c.tr., l.c.tr.) with two
large black patches laterally—without cilia. The pedal tentacles (p.tn.) and membrane between them are black, the lower lip of the pedal gland aperture light yellow, while in front of the proboscis and pedal tentacles the foot is occupied by a semicircular orange-red pad (gl.ft.), strongly ciliated, representing the original sole region of the foot. The mantle margin is encircled by a narrow black line, followed immediately by a band of canary yellow within, and a broader band of scarlet, prominent when the edge of the mantle is everted, passing further back into translucent white. The foot differs from that of Serpulorbis in being surmounted by a large circular operculum (op.) frequently loaded by a hemispherical mass of attached calcareous alga. When the outer surface is clean, it is seen to be thin and horny, shallowly concave, its upturned edge slightly overlapping the margin of the foot. The black pigmentation of the periphery of the foot appears as a dark ring encircling the attachment area of the operculum, while at the centre the disc is raised up by the flat top of a calcareous axial plug, coated thinly with chitin, terminating bluntly above, and projecting strongly below as a round-tipped mamilla deeply inserted into the muscular column of the foot. In Finlay's words, the operculum is “shaped like an everted mushroom.” The structure of the operculum of Novastoa finds its closest resemblances among vermetids in the genus Spiroglyphus. A figure is given for comparison of the operculum of S. megamastus (Plate 8, Fig. 4c) from the Pacific coast of North America. The chief differences from Novastoa are the much wider overlap of the chitinous disc at the sides of the foot, the deeper concavity and the slighter development of the axial plug and mamilla.
The apex of Novastoa lamellosa (Plate 9, Fig. 6) is also strongly reminiscent of Spiroglyphus: in both genera there are two nuclear whorls only, horny-brown in colour, the second much larger and somewhat inflated, covered with fine granulations. In Novastoa lamellosa the lip of the nuclear aperture forms a slight projecting rim around the commencement of the adult shell, which is opaque white in colour, sculptured with close-set, rather indistinct, transverse rugae. The orientation of the nucleus is quite characteristic (Fig. 6b)—its axis directly transverse to that of the adult shell.
Fig. 1 illustrates the structure of the head and foot, with the pallial cavity opened by incision along the dorsal mid-line. The mantle margin is entire in the male, slit backwards along the mid-line in the female for the attachment of egg capsules to the shell as in the serpulorbids. On the left side is a wide triangular notch (inh.) (better developed in some specimens than in others) for the passage of the inhalant pallial current. The osphradium (os.) is a long, straight ridge, the endostyle is absent as typically in Vermetidae. The ctenidium (ct.) is of relatively small size, not larger than in Serpulorbis zelandicus, and, to judge from Yonge's figure, of rather smaller extent than in “Vermetus” novae-hollandiae. As in Serpulorbis zelandicus, the filaments are unspecialized, simple triangular leaflets, showing no tendency to become linear. The ciliary currents, however, especially lateral and frontal, are well-developed, and a stream of water carrying suspended food particles is continuously impelled through the pallial cavity, and particles strained out by passage between the gill filaments are deposited on the ciliated and glandular floor of the pallial cavity
at the right side. Here they are carried forward to the head, by strong ciliary currents along a narrow, rather deeply incised food groove (f.gr.). Material is conveyed to the neighbourhood of the mouth in a long, coherent mucus thread. The transport area is rather better developed than in Serpulorbis zelandicus, where a thin sheet of mucous secretion with embedded particles is carried along a broad, flat food tract. Ciliary currents proceed forward not only in the food groove but also over the whole lateral epithelium of the foot, on both left and right sides converging towards the mid-line on the scarlet pigmented area of the sole. In addition, there are strong currents beating around the periphery of the foot beneath the opercular attachment towards the dorsal mid-line, and wide ciliated tracts beating dorsally at the sides of the foot behind the black lateral patches. It is probable that all of these currents possess some food-gathering function, being freely exposed to alighting of particles when the foot is extended and the pallial current drawn to animal. They no doubt also have a rejectory function, especially the lateral tracts of the foot leading out of the pallial cavity, which carry faecal pellets and introduced carborundum particles to the mid-line of the foot, where enclosed in sheet of mucus they are presently released just below insertion of the operculum (see Fig. 1, f.p.). On the scarlet pigmented disc in front of the pedal tentacles, the mucus from the pedal gland is carried rapidly forward and towards the mid-line. The pedal gland secretion seems to have little connection with the contents of the food groove or with the rejection of waste particles.
Does Novastoa feed by mucus traps, and if so, to what extent does this method supplement ciliary feeding? All members of the Vermetidae (s. str.) possess a well-developed pedal mucus gland. The secretion of the gland does not appear to be employed either for the compaction of food collected by ciliary means, or for the rejection of waste particles. In some vermetids such as Serpulorbis gigas (Boettger, 1930) and Aletes (MacGinitie and MacGinitie, 1949) the gland is very large, and here mucous traps form the exclusive means of feeding. In Serpulorbis zelandicus, considered by the writer (supr. cit.) to engage in both mucus and ciliary feeding, it is also of considerable size. In “Vermetus” novae-hollandiae on the other hand, the gland is much smaller and narrower; in Novastoa (Figs. 1, 2) it is hardly larger, though even here still a very prominent feature, secreting a copious supply of mucus. It is invariably difficult to induce the formation of mucus food traps in the laboratory, but we shall probably not go far amiss in supposing that the great majority—if not all—of the vermetids make some use of the mucous gland in feeding in the natural environment. In none of the vermetids have the pallial organs become highly adapted for the collection of food—even in the ciliary feeding groups the filaments of the gill remain primitively triangular, and an endostylar tract is not developed. It is difficult to regard the mucous gland of “Vermetus” novae-hollandiae, Petaloconchus and Novastoa as undergoing reduction or loss of function. These genera on the contrary show every indication of being the more primitive section of the family, in which the gland probably arose, to be better developed in the more specialized Aletes and Serpulorbis, which have reduced their reliance on the gill. Fig. 1 shows the mucus mass extruded by
the pedal gland of Novastoa lamellosa, a sort of triangular or fan-like sheet of secretion, which could become further spread out, loaded with particles and pulled in by the radula. Novastoa animals during feeding were carefully watched with water goggles. No employment of mucus traps could be observed. The water was rather disturbed by wave action near the surface—it is possible that when water disturbance below the surface is less, mucus feeding may be resorted to without disturbance. Undoubtedly, as claimed by Yonge, mucus feeding is chiefly a habit of still-water vermetids, but the rule has its exceptions, notably Serpulorbis zelandicus, which is able to put out mucus traps in rocky channels through which there is a constant wave surge.
Text Fig. 1—Single rows of radular teeth, the right marginals omitted in each case.
(a) Novastoa lamellosa (Hutton), Poor Knights Is., N.Z. (b) Spiroglyphus
megamastus (March), Cutalina Id., California. (c) Petaloconchus montereyensis
Dall., Carmel Point, nr. Monterey, California. (d) Aletes squamigerus Carpenter,
Newport Bay, California.
The digestive system of Novastoa lamellosa conforms to the general vermetid plan, described by the writer for Serpulorbis. The chief features of comparative importance are the radula (Text Fig. 1, a), and the structure of the stomach and style sac, which form the most complex functional region of the gut.
As with the evidence of the apex and the operculum, the radula affords little doubt as to the close relationship of Novastoa and Spiroglyphus. In Novastoa lamellosa the central tooth is produced posterolaterally into slightly curved, back-directed wings, fitting into concavities in the mesial edges of the laterals. The posterior margin has a median obtuse point, being shallowly emarginate on either side. The anterior edge bears a long median cusp, flanked at each side by a row of three smaller denticles. The lateral tooth has a large cusp the size of the central cusp, with one smaller denticle to the mesial side and a row of three denticles laterally. The inner marginal is equipped with a spur-like forward-projecting cusp a short way behind the tip, with two shorter denticles further back. The outer marginal possesses the sharp, spur-like cusp alone. In Spiroglyphus (Text Fig. 1, b) the postero-lateral wings of the central are stout and peg-like, and the posterior margin is edged with a thin triangular flange. The long median cusp is flanked by four denticles, not three as in Novastoa: a larger denticle flanking the cusp on either side, with an outer row of three short denticles, which may occasionally increase to four. The laterals and marginals are scarcely to be distinguished from those of Novastoa.
The details of the stomach and visceral mass resemble most closely the description by Yonge of “Vermetus” novae-hollandiae. The stomach (Fig. 5) is a large cylindrical chamber, with a shorter style caecum (s.cm.) lying in front. The digestive gland is divided into two parts: a smaller anterior lobe (dg.r.) on the right side, close to the style caecum, and a larger posterior lobe opening from the stomach behind. The posterior lobe (dg.l.) is not greatly elongate and vermiform as in Serpulorbis but shorter, and bluntly tipped, as figured for V. novae-hollandiae, filling the shell tube back to the first septum. The stomach wall is thick, but contains little muscular tissue, being made up of an opaque parenchymatous connective tissue, apparently storing metabolic reserves. The style caecum is separated from the proximal part of the intestine by a single wide, low typhlosole (ty.) running along the ventral side. It opens into the spherical anterior portion of the stomach, the anterior digestive diverticulum being located just beneath the mouth of the caecum. The style rotates against the gastric shield (g.sh.), a transparent recurved plate of cuticle, wide and thin at its margin. The small and rounded gastric shield figured by Yonge in “Vermetus” novae-hollandiae, probably corresponds to the epithelial ridge secreting the actual substance of the shield, or to the strongly projecting fold of epithelium separating the oesophageal opening on the left from the gastric shield. The crystalline style (s.) is retained without resorption through the period between tides when the animals are exposed and feeding is interrupted. It is a narrow, very delicate, flexible rod, small in relation to the size of the animal. Its colour is golden-brown, and diatoms are frequently
contained in the matrix. Apparently a certain amount of food material carried into the intestine becomes caught up in the style substance and carried back to the stomach as the style is thrust downward. A similar “retrieving function” of the style is mentioned by Yonge (1926) in Ostraea, and by the present writer in Serpulorbis (Morton, 1950). In each case the typhlosole is short and the style sac remains widely open to the intestine. A mass of mucus-bound food material is always attached to the posterior end of the style, continuous with or augmented by the food string entering from the oesophagus. The posterior digestive diverticulum (dv.p.) is a wide-mouthed tube opening from the posterior end of the stomach well behind the gastric shield. The ciliary sorting area (s.a.) is on the left side of the stomach, below the intestinal aperture. It is rather small in extent, its size being no doubt correlable with the small size of the style and the relatively pure, well-sorted condition in which diatom material is brought to the stomach. The epithelium is raised into a series of small plicae, the grooves between them collecting rejected particles and empty diatom frustules for carriage to the intestine. At the opening of the intestine the grooves and plicae cease abruptly and the epithelium becomes smooth. The middle intestine (m.i.) passes to the right as a single loop, as in “Vermetus” novae-hollandiae and Petaloconchus montereyensis, separating the renal organ from the anterior lobe of the digestive gland and widening in front into the rectum.
All the vermetid genera with mucus gland and pedal tentacles undoubtedly belong to a natural group, and the evolution of the family is a story of the varying degrees to which mucus trap feeding has been adopted in substitution for the earlier method of ctenidial ciliary feeding. The writer has examined critically the animals of representative species of five genera, namely, Serpulorbis zelandicus, Aletes squamigerus, Petaloconchus montereyensis, Spiroglyphus megamastus, and Novastoa lamellosa. Of these the last three may be placed together, Novastoa and Spiroglyphus especially closely, and Petaloconchus somewhat further removed. All appear to be predominantly ciliary feeders, showing approximately the same degree of development of the ctenidium, pedal gland and pedal tentacles as “Vermetus” novae-hollandiae. The alimentary canal seems to be much alike in all these ciliary feeders, although the material of Spiroglyphus was unfortunately mutilated, so that no accurate dissection of the stomach could be carried out. The strong similarities in the radulae and opercula of Novastoa and Spiroglyphus have already been noticed. An operculum of “Vermetus” novae-hollandiae is figured (Fig. 4, a) showing the same concave chitinous disc as in Novastoa and Spiroglyphus, widely overlapping the foot, but without development of the calcareous plug and mamilla. Radular material of “Vermetus” novae-hollandiae has so far proved unobtainable. The operculum, however, appears to represent a primitive type among vermetids. It resembles the simple concave plate recapitulated in the encapsulate embryo of such species as Serpulorbis zelandicus, which have lost the operculum in the adult. It conceivably gave rise to the Novastoa-Spiroglyphus type of disc, as also to the operculum of Petaloconchus, which is unlike that of any
other vermetids, possessing a sharp, upraised spiral lamella of two whorls. Petaloconchus, although related to the above genera, has several other features setting it some distance apart. In the radula, the postero-lateral processes of the central tooth tend to be long and exaggerated. The female has no median pallial slit for the attachment of the egg capsules, which are retained in a cluster filling the pallial cavity until the emergence of the embryos. In the visceral mass, the anterior lobe of the digestive gland and its diverticulum from the gut have disappeared; the stomach otherwise resembles that of Novastoa.
Somewhat separated from the above genera, and related closely among themselves, are the species of Serpulorbis and Aletes, with which Bivonia is probably associated. This is the mucus-feeding branch of the family. Evolution has evidently proceeded furthest in such forms as Serpulorbis gigas (Yonge and Iles, 1939) and in Aletes. Serpulorbis zelandicus shows a transitional stage with the power of ciliary feeding still well developed (Morton, 1951). Bivonia triqueter—of which the only detailed account appears to be Lacaze-Duthier's memoir (1860)—is stated by Yonge to be a mucus trap feeder. Aletes squamigerus according to MacGinitie and MacGinitie, 1949, “secretes mucus that extends upward in the water as a triangular sheet. This sheet of mucus is allowed to float and wave in the water for a while, then the animal pulls it down and eats it, with what food material has adhered to it. When (the animals) occur in clusters, the fan-shaped sheets of mucus they put out become entangled, and the table with its bill of fare becomes a community affair. When one member in such a group begins to eat the mucus sheet all the others start swallowing. The sheets of mucus may extend upward into the water for five or six inches, and the upper edges may be somewhat frayed.”
The radulae of Serpulorbis zelandicus (see Morton, 1951), Serpulorbis gigas and Aletes squamigerus (Text Fig. 1, d), the type species of Aletes, are only with difficulty distinguishable. From data supplied by Dr. Keen, the apex of Aletes is almost exactly like that described for Serpulorbis zelandicus (Morton, 1951). The nucleus is three-whorled, translucent white, and the pitted sculpture of the first two or three adult coils is alike in both genera. Both groups have lost the operculum in the adult, while Bivonia retains it as a tiny vestige at the centre of the foot. Since adult sculpture is unreliable, it may be doubted whether there are any features accessible to the conchologist whereby Aletes and Serpulorbis may be safely separated. In the animal of the various species there may be quite considerable adaptive differences—for example, in the relative development of the pallial organs of Serpulorbis zelandicus and Serpulorbis gigas.
In the Vermetidae especially, an appreciation of the structure and biology of the animal is necessary for a proper understanding of the group. For taxonomic convenience it is desirable, however, that the final arbiter in separating genera should be a conchological character. Radulae tend to be rather conservative as between genera; perhaps their most useful variation is in the shape of the central tooth. Opercula, when present, form a reliable natural character. The most valuable evidence in determining genera will probably be yielded by the nuclear form and sculpture; Dr. Keen is at present completing
Fig. 1—Novastoa lamellosa. Head, foot and pallial region, showing course of ciliary
currents in relation to feeding and cleansing. The mantle is incised along the
dorsal mid-line and reflected to the left. The extent of the pedal mucus gland is
indicated by the broken line.
Fig. 2—Novastoa lamellosa. Longitudinal section of the head and foot, slightly to the
right of the sagittal plane. Operculum removed. As compared with Fig. 1, some
obvious contractions of the pedal structures have been brought about by fixation.
The groove along the mesial edge of the pedal tentacle is indicated from the
lateral aspect by broken lines. A.C.T.R., anterior ciliated tract of foot; CPH.,
head; CT., ctenidium; CT.F., ctenidial filament; F.G.R., food groove; F.P., faecal
pellet; GL.FT., G.L., glandular tract of foot; H.GL., hypobranchial gland;
INH., inhalant notch in mantle: L.C.TR., lateral ciliated tract of foot; M., mouth;
M.D., duct of mucus gland; M.GL., pedal mucus gland; MUC., mucous trap
secreted by gland; OD., odontophore; OP., operculum; INS., insertion of opercular
mamilla; OS., osphradium; P.TN., P.T., pedal tentacle; RAD., radular caeccum;
RM., rectum; SL., salivary gland.
Fig. 3—Petaloconchus montereyensis. Operculum from upper surface.
Fig. 4—Diagrammatic views of opercula, from upper surface (above) and in schematic
section through foot (below). (a) “Vermetus” novaehollandiae, (b) Novastoa
lamellosa, (c) Spiroglyphus megamastus. Encrusting coralline algae have been
Fig. 5—Novastoa lamellosa. Semidiagrammatic longitudinal section of the anterior part
of the visceral mass, showing internal structure of the stomach and style caecus.
DG.L., portion of left lobe of digestive
gland: DG.R., right lobe of digestive gland: DV.A., anterior (right) digestive diverticulum; DV.P., posterior (left)
digestive diverticulum; G.SH., gastric shield; M.IN., middle intestine; OES.,
oesophagus; P.IN., proximal portion of the Intestine; S., crystalline style;
S.A., ciliary sorting area; S.CM., crystalline style caecum; TY., typhlosole.
Fig. 6—(a) Novastoa lamellosa. Nucleus with initial portion of adult shell. From an
embryo within the pallial cavity. (b) Novastoa lamellosa. Nucleus attached to
earlier portion of the adult shell.
a review of both adult and embryonic conchological features. Within generic limits, coloration of the animal, as well as details of nuclear sculpture, is often a useful interspecific difference. The generic assignation of the known vermetid species is likely to repay careful conchological investigation; in the absence of animals or nuclei there has been a tendency to relegate species uncritically to a few widely used generic categories. Vermicularia, which has been a favourite hold-all in the past, is not a member of the Vermetidae (s. str.) at all; a figure of the nucleus forwarded by Dr. Keen confirms its recent location by the writer (Morton, 1951a) in the Siliquariidae.
References to Literature
Boettger, C. R., 1930. Studien zur Physiologie der Nahrungsaufnahme festgewachsener Schnecken. Die Ernährung der Wurmschnecke Vermetus. Biol. Zbl., Bd. L, 581–597.
Cranwell, Lucy M., and Moore, Lucy B., 1938. Intertidal Communities of the Poor Knights Islands, New Zealand. Trans. Roy. Soc. N.Z., 67 (4), 375–407, pls. 53–54.
Finlay, H. J., 1928. The Recent Mollusca of the Chatham Islands. Trans. N.Z. Inst., 59 (2), 232–286.
Hutton, F. W., 1873. Catalogue of the Marine Mollusca of N.Z., 30.
Lacaze-Duthiers, H. de, 1860. Mémoire sur l'anatomie et l'embryologie des vermets (Vermetus triqueter et V. semi-surrectus Phil.). Ann. Sci. Nat. Zool. (4), xiii, 209–296.
MacGinitie, G. E., and MacGinitie, Nettie, 1949. Natural History of Marine Animals. McGraw-Hill Book Company Incorporated, New York, p. 366.
Morton, J. E., 1951. The Structure and Adaptations of the New Zealand Vermetidae. I. The Genus Serpulorbis. Trans. Roy. Soc. N.Z., 79 (1), 1–19.
—— 1951a. The Structure and Adaptations of the New Zealand Vermetidae. II. The Genera Stephopoma and Pyxipoma. Trans. Roy. Soc. N.Z., 79 (1), 20–42.
Yonge, C. M., 1926. Structure and Physiology of the Organs of Feeding and Digestion in Ostraea edulis. J. Mar. Biol. Assoc. U.K., 14, 295–386.
—— 1932. Notes on Feeding and Digestion in Pterocera and Vermetus, with a discussion on the occurrence of the crystalline style in the Gastropoda. Sci. Repts. G. Barrier Reef Exped. Brit. Mus. (Nat. Hist.), I, 259–281.
——, and Iles, E. J., 1939. On the Mantle Cavity, Pedal Gland and Evolution of Mucous Feeding in the Vermetidae. Ann. Mag. Nat. Hist., 3, 536–555.