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Volume 79, 1951
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The Structure and Adaptations of the New Zealand Vermetidae
Part I. the Genus Serpulorbis

[Read before the Auckland Institute May 17, 1949; received by Editor, February 1, 1950.]

Contents

Taxonomy.

Serpulorbis zelandicus, S. aotcaroa n.sp., Novastoa lamellosa.

Serpulorbis zelandicus.

I.

Biology and Feeding Mechanism.

II.

The Alimentary Canal.

III.

Reproductive System.

Discussion.

Summary.

References to Literature.

The Vermetidae are at present one of the least satisfactorily known of gastropod families. The shell is typically spiral in the embryo, but at an early stage the spire loosens, becoming finally wholly or partially uncoiled and often irregularly twisted, either embedded in or cemented to the substratum. Adult shell characters are unreliable in classification and the group holds no delights for the conchologist: a natural arrangement depends principally on the structure of the animal, which is intimately adapted to the specialised mode of life. It is now proposed to consider the structure of the New Zealand representatives of the Vermetidae from the functional aspect, as a contribution to the classification of the family.

The principal literature dealing with the vermetid animal consists of the early memoir of Lacaze-Duthiers (1860) on the anatomy and embryology of “Vermetus” triqueter and “Vermetus” semisurrectus; a short account by Rougement (1880) recording the mode of feeding of “Vermetus” gigas; and in more recent times three papers on the biology of the Vermetus group—Boettger (1930) on V. gigas, Yonge (1932) followed by Yonge and Iles (1939) comparing V. novae-hollandiae and V. gigas.

Powell's check-list of the New Zealand recent mollusca (1946) includes two species assigned to the genus Vermicularia—sipho (Lamarck, 1818) and maoriana (Powell, 1937). The latter is a deep-water Aupourian form, known at present only from the type shell; as its author points out, the generic location in Vermicularia is to be regarded as merely provisional. The New Zealand species associated with the Australian sipho may be conveniently restored to Serpulorbis as in Suter's Manual (1913); Finlay (1927), in following Hedley's removal of these shells to Vermicularia, had no material of the animal available. As defined by Thiele (1931) Vermicularia is not appropriate for sipho; that genus is best reserved for vermetids with the “shell not

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attached, at first coiled like Turrietella, later irregularly extended.” Moreover, there is an “operculum as large as the aperture.” Serpulorbis is distinguished in being firmly attached along one side to the substratum, and irregularly uncoiled from the outset. The operculum is always absent as in sipho.

Taxonomy

When the systematics of the sipho group of vermetids comes to be fully worked out, the most reliable specific characters will probably be found to be the coloration of the animal and the dentition, as well as the form and sculpture of the nuclear shell. Collections of vermetids should always include if possible colour records of the living animal, as well as preserved material from which radulae and embryos can be obtained. The adult shell sculpture in species of Serpulorbis appears to be in many cases almost identical, and the conchological features show little well-marked difference; the disposition of the whorls is highly irregular according to the shape of the substratum. For the purposes of the present work, which is intended primarily as an account of the structure of the animal, material of Serpulorbis was studied from two Auckland localities, Milford Reef on the eastern shore of Rangitoto Channel, and Otata Island in the Noises Group, five miles north of Rangitoto Island. The Milford material hitherto assigned to sipho yields two clearly defined species, readily distinguishable on the basis of animal coloration and dentition, but apparently without clear-cut conchological differences.

On general grounds it is highly unlikely that the Australian sipho is, properly speaking, represented in New Zealand. Serpulorbis, like other vermetid genera, is a group in which a high degree of speciation is to be expected, both from the sessile habit of the organism and from the mode of reproduction. The free-swimming veliger larval stage is entirely eliminated, and the eggs are retained in thin capsules attached to the inside of the parent shell. There is thus no effective means of transport either of larvae or adults, and the embryo after emergence can wander about for only a relatively short distance before settling in its attached position. The sessile habit renders current fertilization necessary, and eggs are probably fertilized normally by sperm from animals only a few metres distant, a further factor tending to bring about reproductive isolation. It is thus seldom to be expected that single species of vermetids will be represented on both sides of the Tasman. For example, the neozelanic shells ascribed to Lilax nucleogranosum are shown in a forthcoming paper to be separated by valid differences from Verco's South Australian species. Pyxipoma weldii appears to be the only New Zealand exception to this rule, proving to be identical with the Australian and Tasmanian species; this fact may be accounted for by the better facility of distribution in the siliquariids, which are found embedded in buoyant masses of sponge capable of being carried long distances by currents.

In comparing the neozelanic serpulorbids with sipho regard may be had to the existence in the New Zealand “Miocene” of ancestral forms to the present-day species (“Yermicularia” ophioides and “V.” lornensis) [see Finlay, 1927, p. 386] at an era when Australian and New Zealand molluscan relationships were admittedly remote. The sculpture of the adult New Zealand shells is in general close to

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Serpulorbis zelandicus
Fig. 1—Head, foot and pallial cavity of female. The dorsal body wall has been removed
to show the anterior part of the alimentary canal and the pedal mucus gland.
Fig. 2—Diagrammatic view of the head and foot, from above, partly withdrawn into
the shell.
Fig. 3—Diagram of the opening of the pedal mucus duct, pedal tentacles and glandular
tract of the foot. The pedal are shown contracted to approximately half their length.
Fig. 4—Diagrammatic transverse section of pallial cavity and trunk region.
Fig. 5—Transverse section of pedal mucus gland and its duct.
Fig. 6—Transverse section of small portion of mucus gland and two of its smaller ducts.
Fig. 7—Gill filaments in transverse section.
Fig. 8—Terminal region of gill filament, showing arrangement of cilia.
AB.C, abfrontal cilia; A.FT, anterior margin of sole; APC, apical cilia; BL.S,
blood sinus; CT, gill: EXH, exhalant opening of pallial cavity; FR, frontal cilia;
F.T, food tract; G.FT, glandular sole of foot; HY.G, hypobranchial gland;
INH, inhalant opening of pallial cavity; LT.C, lateral cilia; MO, mouth;
MC, mucus cells; MD, median longitudinal duct of mucus gland; MD', Finer ducts
of mucus gland in transverse section; MD”, finer ducts of mucus gland in
vertical section; OE, oesophagus; OS, osphradium; PA, mantle; PD.T, pedal
tentacle; P.G, pedal mucus gland; PG.O, opening of pedal gland duct; PH,
pharynx; R, radula; R.CM, position of tip of radular caecum; R.EP, respiratory
portion of gill; RM, rectum; SK.R, skeletal rod of gill filament; TE, cephalic
tentacle; T.FT, terminal disc of foot.

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Serpulorbis zelandicus
Fig. 9—Stomach and crystalline style caecum, opened from the right side, showing the
course of the ciliary currents.
Fig. 10—Intestine, renal organ and female genital ducts, viewed from the right side.
The renal organ has been opened to show the course of the middle intestine.
Fig. 11—Diagrammatic transverse section of the female genital ducts passing through
the albumen gland and
the receptaculum.
Fig. 12—Diagrammatic transverse section of the capsule gland.
Fig. 13—Portion of terminal lobule of the digestive gland, showing digestive and excretory cells.
Fig. 14—Portion of hypobranchial gland, showing ciliated cells and two types of
glandular cells.
Fig. 15—Portion of epithelium of food tract, showing ciliated and mucus gland cells.
Fig. 16—Portion of epithelium of the capsule gland.
A.CH. anterior chamber of stomach; A.DIV, anterior digestive diverticulum;
CIL.C, ciliated cell; CPS, capsule gland;
CPS', ventral opening of capsule gland; C.ST. crystalline style: DI.C. distal pseudopodial portion of digestive cell;
DI.C, digestive cell with absorbed particulate material; DI.C”, basal portion
of digestive cell, containing greenish spherules before egestion; F, S-shaped fold
of stomach sorting epithelium; EX.C. excretory cell; EX.SPH, excretory spherule;
G.SH, gastric shield; GL.C”, GL.C”, glandular cells of hypobranchial gland;
HY.GL, hypobranchial gland; M.INT, middle intestine; MU.C, mucus gland cell
of food tract; OES, oesophagus; OV.D, ovarian duct; P.CH, posterior chamber
of stomach; P.DIV, posterior digestive diverticulum; P.INT, proximal portion
of the intestine: R.DIG, right (anterior) lobe of digestive gland; REC, receptacu
lumseminis; REN. renal organ; RM, rectum; S.A, ciliary sorting area of stomach;
S,CM, style cnecum; ST, crystalline style: V.TY, ventral typhlosole,

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Fig. 17—Serpulorbis sipho (Lamk.). Mature shell. South Australia.
Fig. 18—Serpulorbis sipho (Lamk.). Juvenile shell after attachment with two spiral
turns. South Australia.
Fig. 19—Serpulorbis aotearoicus n.sp. Egg capsules attached to inner surface of shell
tube of female.
Fig. 20—Serpulorbis aoteoroicus n.sp. Embryo at stage of single-whorled shell. 0.3 mm.
in diameter, removed from capsule.
Fig. 21—Serpulorbis aotearoicus n.sp. Velate embryo at later stage, removed from
capsule, with 1 ½-whorled shell, 0.5 mm. in diameter.
Fig. 22—Serpulorbis aotearoicus n.sp. Embryo at crawling stage, removed from the
mouth of the adult shell tube. The creeping surface of the sole and the concave
operculum are fully developed, and the velum lost.
Fig. 23, 24—Serpulorbis aotearoicus n.sp. The embryo shell at the stage represented in
Fig. 6.
Fig. 25—Serpulorbis sp. Juvenile shell, at similar stage to Fig. 2, Unassigned to species,
Off Oamaru, 50 fathoms, Finlay Collection.

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sipho. As regards the juvenile shell just after attachment, South Australian specimens identified by Mr. B. C. Cotton as belonging to sipho are quite dissimilar in sculpture to young Serpulorbis at the same stage from New Zealand localities. Fig. 25 illustrates a juvenile shell dredged off Oamaru (Finlay Collection). In the adult the sculpture of the young shell is usually obscured by the superposition of the adult coils, and it was not possible to assign the dredged material conclusively to its species. Finally, the coloration of the animal of sipho, illustrated by Quoy and Gaimard (1834) under the name Vermetus arenarius, is again unlike that of either of the New Zealand animals studied. It is therefore proposed to designate the Auckland species of Serpulorbis as follows:

1. Serpulorbis zelandicus (Quoy and Gaimard) 1834

The first of the Milford species is clearly entitled to the original name proposed by Quoy and Gaimard for their neozelanic vermetid. The authors gave no description of the shell characters, merely remarking upon what has been pointed out above—the similarity of the shell to related species of Australian vermetids, and proceeding to describe the coloration of the animal. “Ce vermet a tellement de rapports avec V. elegans, que c'est avec doute que nous en faisons une éspèce particulière. On ne peut qu'indiquer ses couleurs. Těte jaunâtre en arrière, brun et ponctuée de rouge en avant. Le pied est seulement jaunâtre avec des taches rouges. Le manteau est largement bordé d'un orange vif. Le tube, contourné sur lui-même, ne nous a point offert de caractère appréciable sur le moment.” Reference to Quoy and Gaimard's atlas of zoological illustrations shows that the two coloured drawings of Vermetus zelandicus quite adequately identify the animal with one of the Milford species under consideration. The use of the term zelandicus was first confused by Suter, who quite unwarrantably annexed to Quoy and Gaimard's colour description an account of the shell of Hutton's Siphonium lamellosum.

Finlay (1930) thereupon rightly considered Hutton's shell to be identical with that described by Suter under Serpulorbis zelandicus, and was led to accord the latter name priority over lamellosum for Hutton's Siphonium. Serpulorbis zelandicus, however, applies validly to a separate shell, prior to and quite distinct from Hutton's species, and a fuller description is now provided.

The shell is moderately large and vermiform, sub-solitary or in small groups of two or three intertwined, seldom forming larger aggregates. The coiling is completely untwisted and the disposition of the whorls irregular or in two or three loosely coiled convolutions attached along the whole of one surface to the substratum, save for the apertural portion which is generally vertical so that the opening faces directly upwards. The aperture is circular in section, the attached sides of the tube flattened or irregularly moulded to the substratum, and the exposed surface regularly convex. The diameter regularly increases, reaching 8–10 mm, across the aperture of a large shell. The sculpture of the free surface is predominantly of longitudinal ridges of somewhat unequal size, consisting of several more prominent cords, separated by three to seven rather smaller riblets. The longitudinal sculpture is crossed at close intervals by small sharp rugae sometimes giving a finely scaled appearance to the living shell, and in the beach worn

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shell, intersecting the ribs to give a distinctly moniliform or tessellated ornamentation. Towards the aperture the tube is frequently thin and sharp-edged, with growth striae forming the only sculpture. The colour of the shell is yellowish-brown, usually encrusted or eroded in the earlier portions, and often becoming orange-brown or reddish, frequently with tints of purplish towards the aperture. The interior is shining and porcellanous, usually white, though often purplish-brown. The earlier portions of the shell, contrary to Suter's statement, are septate, being cut off by thin calcareous partitions, deeply concave aperturally, at distances of 5 mm.–6 mm. apart.

The animal is handsomely pigmented, the head and foot regions being most frequently orange-red in colour, but varying a good deal through light-brown, yellowish to a deep chocolate red. The disc and sides of the foot are finely sprinkled with yellow and red, the cephalic shield being reddish-brown, and the tip of the proboscis always darker brown. A broad line of alternating brown and yellow patches runs along the side of the cephalic shield as far as the tentacle base, and both the mantle rim and the margin of the pedal disc are likewise ornamented with a brown and yellow band. The viscera are yellowish-white in colour, the digestive gland jet black. There is no opereulum in the adult.

The radula (Text-Fig. 1) closely resembles that of Serpulorbis gigas as depicted by Troschel (Das Gebiss der Schnecken). The central tooth is very wide, twice as broad as long, with a long triangular median cusp reaching almost to the posterior margin, and three much smaller lateral denticles. The laterals are wide, deeply excavated from the outer edge, with a long cusp at the mesial edge and three or four

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Text Fig. 1—Single row of radular teeth of Serpulorbis zelandicus (Q. and G.). Text Fig. 2—Radular teeth of Serpulorbis aotearoicus n.sp.

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blunt denticles laterally. The two pairs of marginals are stout, curved and falciform with small blunt denticulations along the convex edge.

Slide of radula and colour records of living animals are deposited in the Auckland Museum.

2. Serpulorbis aotearoicus sp. nov.

This species resembles zelandicus in adult shell characters, tending to a darker brown or purplish-brown colour, but without apparent difference in sculpture. The coloration of the animal is, however, quite distinctive. The predominating colour of the head, proboscis and foot is black, or at times a lighter shade of greyish, lightening to pinkish-brown further back on the trunk and proximal parts of the foot. The tip of the proboscis is always deep black, and along the margin of the cephalic shield is a bright yellow or yellowish-cream line, passing backwards from the base of the short cephalic tentacle, which is itself yellow. A margin of the terminal pedal disc forms a broad band of yellow, and the black pedal disc itself is closely flecked or striated with yellowish. The pedal tentacles are mottled with black and yellow. The mantle is light brown in colour, its border edged widely with bright orange-red. The viscera are pure white, the digestive gland black. The opereulum is absent in the adult.

The radula (Text-Fig. 2) is distinguished from that of zelandicus by the narrowness of the central and lateral teeth, which are slightly greater in length than in breadth. The principal cusp of the central tooth is much smaller than in zelandicus, usually not more than a quarter the length of the tooth. The basal wings of the central tooth are shortly truncated, and the lateral teeth set closer to the mid-line. Their mesial cusps are also much smaller than in zelandicus.

The embryo shell of aotearoicus is illustrated in Figures 20–24. The nucleus is of one and a half whorls, with the apex a transparent spherical bulb, soon developing a finely lirate longitudinal sculpture, crossed by very delicate accremental striae. The lirate portion of the nucleus gives place abruptly to a second whorl which is without longitudinal sculpture, though marked with fine, distinct growth striae, and tinted with several longitudinal splashes of reddish-brown. The aperture at the one and a half whorled stage is approximately triangular, slightly notched or effused at the inner lip. The shell is at this stage imperforate, but the coiling early begins to loosen up, and the earliest part of the attached shell has the appearance of Fig. 25, somewhat resembling a worm tube of Spirorbis. The embryo at the free-moving two-whorled stage has a large chitinous operculum, concave or saucer-shaped, with the margin simple.

Type in the Auckland Museum. Slide of radula and colour record of living animal are deposited with the type. Paratypes in the Zoology Museum, Auckland University College.

Localities. Milford Reef, Rangitoto Channel (type)—under boulders at low spring tide, on outer fringe of reef, together with smaller numbers of zelandicus; Otata Island, Noises Group, on exposed rocks, covered with Melobesia at low spring tide.

Complete correlation was found in radula and animal coloration for 50 specimens of aotearoicus and 16 specimens of zelandicus examined from Milford. The two species occur intermixed in the same locality,

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although under a single boulder one or other form is usually found alone, aotearoicus tending to predominate in numbers. The separation of species of Serpulorbis without available shell differences unfortunately complicates the task of the systematist. An extended survey of New Zealand material of this genus may however bring to light a speciation pattern of compensating interest. Especially valuable would be data from southern and outlying portions of New Zealand on the radula and coloration of Serpulorbis. The Finlay and Powell Collections contain juvenile shells of the recently attached stage of Serpulorbis, which are not yet able to be assigned to their appropriate species. They are certainly different from corresponding sipho (Fig. 18). The attached surface is flat and smooth, the exposed surface with four very distinct spiral cords, connected by strong lamellae crossing the intervening grooves so as to give a regularly pitted appearance to the sculpture.

Genus Novastoa

Novastoa lamellosa (Hutton) 1873

Hutton's specific name must stand. As explained above, Suter incorrectly applied Quoy and Gaimard's name zelandicus to shells of Hutton's species, leading Finlay to accord it priority over lamellosum. As Finlay points out (1930), “zelandicus” shells in Suter's sense are identical with Hutton's lamellosum, save for the lack of an operculum. The shell usually forms massed aggregations, while occasional specimens are found with the long, straight distal portion, which gave rise to Suter's description of “zelandicus.” In both cases the sculpture is identical, with strong, transverse rugae. No doubt Suter was dealing with beach-worn shells in which no opercula remained; the shells labelled “2405 Serpulorbis zelandicus, Bay of Islands.” in the Suter Collection are certainly Novastoa. Suter's figure of Serpulorbis zelandicus is not easily recognizable. Hutton's description of the operculum in lamellosum as hemispherical is easily understood, being evidently due to the presence of the usual dome of encrusting coralline on its free surface, rather than as suggested by Finlay to confusion with a loose septum. The occurrence and ecology of Novastoa lamellosa have been well discussed by Cranwell and Moore (1937) in their account of the inter-tidal zonation of the Poor Knights. An account of the structure and biology of this form is now in preparation by the present writer; its closest affinities among New Zealand vermetids are evidently with Serpulorbis.

Serpulorbis zelandicus I. Biology and Feeding Mechanism

Both Serpulorbis zelandicus and S. aotearoa are typically clean-water species, appearing to prefer at least a moderate amount of wave surge. They are accessible at Milford Reef only at extreme low-water spring tide; they may be found on exposed rock surfaces, heavily encrusted with corallines, but generally avoid direct light, the shell being attached to the undersides of loose boulders or to holdfasts of Ecklonia. At Milford Serpulorbis is confined to the extreme seaward fringe of the reef, where the water is clear, and the bottom consists of hard rock, or of coarse-grade shell sand. Both species are much less tolerant than the opereulate vermetid Stephopoma of the presence of finely divided detritus and silts.

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Externals and Pallial Organs

When the animal is fully extended, the margin of the mantle forms a circular rim reflected over the edge of the shell, and the head and foot can be moved actively about, with the pair of long pedal tentacles directed upwards. The foot (Figs. 1, 2) is truncated and plug-like, its circular disc almost completely occupying the aperture. Lacking an operculum, the animal is able to withdraw far into the shell tube for protection by contraction of the slender columellar muscle. The edge of the foot is usually indented on either side by a small channelled lip, serving for the passage of the inhalant pallial current on the left and the exhalant on the right. This probably gave rise to the statement by Quoy and Gaimard, repeated by Suter, that the foot is cruciform. Dorsally to the pedal disc, the aperture is occupied by the flattened, broadly ovoid cephalic shield, terminating in a short, wide proboscis (Fig. 1) with the slit-shaped mouth at the tip. The cephalic tentacles (Fig. 2, TE.) are short and blunt, with minute eyes at their outer bases.

The organs of the pallial cavity (Fig. 4) fairly closely resemble those of “Vermetus” novae-hollandiae, described by Yonge (1932) and Yonge and Iles (1939); the animal is without doubt predominantly a ciliary feeder, employing the ctenidium for the collection of food particles—principally benthic diatoms and fine detritus of plant origin. The ctenidium (Figs. 1, 4, CT.) has about the same extent as in “Vermetus” novae-hollandiae and is thus much better developed than in Serpulorbis gigas, which feeds by the mechanism of mucus strings. The mantle margin forms an entire skirt in the male without trace of inhalent siphonal process; the female shares with other species of “Vermetus” and applied groups the longitudinal slit along the dorsal mid-line of the mantle along which the row of egg capsules is attached to the inner surface of the shell. The ctenidium lies to the left of the mantle cavity, and to the right of the slit in the female lies the hypobranchial gland (Fig. 4, HY.G.), which extends across the pallial (ventral) aspect of the rectum. The gill filaments (Figs. 7, 8) are typically triangular, with the frontal side supported by strong skeletal rods. These form a framework extending from the axis across the ventral aspect of the ctenidium, from which the wide respiratory lamellae pass dorsally to the mantle wall. According to Yonge's figure (1932) the filaments in “Vermetus” novae-hollandiae are much narrower and more elongate. As Yonge points out (1938) the filament width tends to be reduced by shrinkage of the respiratory area in the preserved material figured. This fact, however, does not appear to account sufficiently for the difference; in Serpulorbis zelandicus fixation did not cause appreciable shrinkage, and the filaments appear rather thick and non-membranous. No doubt, in “Vermetus” novae-hollandiae, the narrowing of the filaments provides a further adaptation to ciliary feeding, by which this species obtains the whole of its nutriment. In Serpulorbis zelandicus, on the other hand, it is clear that ciliary feeding may occur with retention of the primitive shape of the filaments, though in most ciliary feeding prosobranchs the filaments are either wholly linear (Crepidula, Orton, 1912; Stephopoma, present writer, in manuscript) or partially rod-like (Turritella, Graham, 1939; Struthiolaria, Morton, 1951). Conversely, in such genera as Strombus and Xenophora (Morton, 1949) the more general

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mode of feeding may be retained, while the filaments become elongate in adaptation to cleansing requirements on a silty bottom.

The ciliation of the gill in Vermicularia zelandicus shows the usual three tracts, frontal, lateral and abfrontal, and the bluntly rounded tip of each filament has a dense tuft of long apical cilia, 18μ in length. The frontal cilia are especially robust and carry a strong current to the tips of the filaments, bearing particles sifted out of the respiratory current by the lateral cilia, which are unusually long (20μ) and lash-like, beating upwards between the gill filaments towards the abfrontal side. The respiratory surface of the filament is wide, with dense unicellular mueus-producing glands, and small cuboidal or flattened epithelial cells, the cilia but sparsely developed. The abfrontal current is rather feeble, carrying to the tips of the filaments such smaller particles as may have passed across the lateral tracts with the respiratory current. The gill incompletely divides the pallial cavity into left inhalant and right exhalant chambers. The right chamber is equipped with two broad longitudinal zones of glandular epithelium, the hypobranchial gland forming its roof and the food tract running along the floor. The copious mucus supply of the pallial cavity is thus produced almost wholly in the exhalant chamber.

The hypobranchial gland forms a broad flat sheet. It is never deeply rugose as in the majority of prosobranchs, and it is probably replaced functionally to some extent by the glandular epithelium of the food tract. It is composed (Fig. 14) of narrowly constricted ciliated cells (90–100μ tall) with elongate, rod-shaped nuclei, and glandular cells of two kinds. The first is distended and vesicular, its secretion staining lightly with iron haematoxylin; the second is about twice as numerous, each cell containing up to 100 secretion spherules, staining orange-brown with Van Giesen's. The floor of the mantle cavity is divided along the mid-line by a narrow, more or less distinct ridge (see Fig. 4) upon which the tips of the gill filaments frequently come to rest in life. Food particles are deposited on the food tract immediately to the right of the median ridge. They are at once carried away from the gill, and swept forward by the long food-tract cilia. In addition, the apical ciliary tufts of the gill appear to have a forward beat, though so far as can be ascertained when the mantle is intact, the filaments are not of sufficient length to allow these tufts to work together with the food tract cilia, as Yonge and Iles (1939) state is the case in “Vermetus” novae-hollandiae. The food tract corresponds to the groove as described in other ciliary feeding gastropods; it has however no muscular marginal folds, and never forms a temporary tube in which mucus strings could be moulded. The food-tract epithelium (Fig. 15) is tall (90μ) and there are two types of cell—ciliated cells with the ciliary coat well developed (12μ) and long cigar-shaped glandular cells, whose secretion stains deeply with haemotoxylin.

Mucus gland cells are retained on the gill filaments, but have probably an entirely lubricating and cleansing function: from their position they can have little to do with food collecting. The gill axis has no endostylar zone; while the whole pallial epithelium is diffusely glandular, there is no aggregation of specialized ciliated and mucus cells into a well-marked tract as in Crepidula, Turritella and Struthiolaria, as well as in the vermetid Stephopoma. Thus, in Serpulorbis

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zelandicus, almost the whole of the food-collecting mucus is derived from the cells of the food tract, where the epithelium is specialised, in marked contrast to its normal development in those types in which an endostylar mucus supply is available. The osphradium in Serpulorbis zelandicus is typically developed—a simple linear ridge, non-pectinate, meandering along the whole length of the gill axis.

The Foot and Pedal Gland (Figs. 3, 5, 6)

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.).

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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.

II. The Alimentary Canal

The Vermetidae—as shown by Yonge (1932)—are among those prosobranchs that have developed a crystalline style, in correlation with the mode of continuous feeding on fine particles. The foregut is a simplified region producing an abundant mucus supply by which food is carried back to the stomach. In the specialized stomach region the food particles are stirred and subjected to preliminary digestion by the style, and sorted by ciliary action, after which assimilable material is passed into the digestive diverticula. As usual in current-feeding animals, the intestine is devoted to the formation of firm faeces.

The mouth in Serpulorbis zelandicus is a vertical slit, equipped with a pair of cuticular jaws, whose edges diverge below on either side of a protrusible odontophore. The apposable margins of the jaws are strengthened by a row of chitinous denticles, each secreted by a single underlying columnar cell; they are powerful enough to take a firm grasp of a needle placed in the buccal cavity. As the mucus-

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bound food material is picked up from the foot by the jaw-plates, the radular teeth are erected, and their sharply pointed cusps rake the food inside the mouth. Here it is passed backward by ciliary currents along the glandular dorsal region of the pharynx. The pharynx (Fig. 1, PH.) has the usual structure—a stout ovoid bulb with an odontophore supported by paired cartilages. The radula caecum is small, projecting only a short distance through the pharynx floor. Its recurved tip (Fig. 1, R.CM.) rests just beneath the base of the oesophagus. Tiny paired salivary glands (Fig. 1, S.G.) open through the pharynx roof at the beginning of the oesophagus. They are visible in dissection as diffuse whitish lobules, and their tubules are without lumina, with mucous cells containing a light-staining secretion, but with no trace of enzyme-producing cells. The ducts are short and narrow (150μ) with ciliated cells sparsely distributed between stouter gland cells.

The oesophagus widens behind the nerve ring to form a spacious, thin-walled tube (1 ½mm. in diameter) with impermanent longitudinal folds. Two ridges are, however, more constant, bounding the dorsal food-conducting tract leading from the pharynx and passing down the left side of the oesophagus at the site of visceral torsion. The food tract finally passes backwards along the oesophageal floor, remaining distinct with taller lateral folds for two-thirds the distance to the stomach. Posteriorly the oesophagus becomes narrower (½mm.) and its epithelium is thrown into a series of more permanent folds, with very distinct ciliary currents leading along the summits towards the stomach. As in other style-bearers there remain no traces of the ventral glandular pouches of the anterior oesophagus. The cells of the morphologically dorsal food tract are taller (50μ), but the whole lining epithelium is uniformly ciliated and glandular, with mucous cells staining lightly with haematoxylin, and forming by their secretion the oesophageal food string.

The Stomach and Crystalline Style Caecum

The stomach in Serpulorbis zelandicus is a stout, obtusely angled sac with anterior and posterior limbs. The anterior portion is continuous in front with the style caecum and intestine, and the posterior receives the oesophagus on the left and gives exit behind to the posterior digestive diverticulum (Fig. 9, P.DIV.). A second, much smaller, diverticulum (A.DIV.) opens anteriorly, immediately below the mouth of the style caecum. The angle of the stomach is dilated into a spherical chamber, lined with cuticle, in which the head of the style rotates, bearing against a thin, curved plate of cuticle forming the gastric shield (G.SH.). The style caecum (S.CM.) is wide and cylindrical, slightly tapering anteriorly, and lined with tall transverse folds of ciliated epithelium.

The cilia—as typically in this location—are extremely long and robust, equal in length (15μ) to half the height of the cells. Their rapid transverse beat brings about a clockwise rotation of the style. The caecum is in wide communication with the proximal part of the intestine, being bounded by two narrow typhlosoles which remain adpressed to the style in life. The dorsal typhlosole is clad with short eilia, beating backwards towards the stomach and imparting a down-

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ward thrust to the style. Along its summit is a tract of columnar cells with darker staining contents, comparable with the style secreting zone in Struthiolaria (Morton, 1951). The ventral typhlosole also possesses short cilia, though its currents—while probably towards the intestine—are indistinct. To the left of the style caecum, a rapid forward current is maintained by the intestinal ciliated epithelium. The crystalline style (C.ST.) in Serpulorbis zelandicus is 4–5 mm. in length, relatively less stout than in “Vermetus” novae-hollandiae, and noteworthy for its delicate semi-fluid structure. It dissolves rapidly on cessation of feeding or when removed from the animal. The gastric end of the style generally continues directly into a mucus string containing ingested food material—chiefly diatoms—drawn out of the opening of the oesophagus. In contrast with Struthiolaria (Morton, 1951) there is no broad protective typhlosolar flange enwrapping the style, and its whole substance is often permeated with finely divided food particles. These appear to be swept into the caecum from the intestine and caught up in the viscid style substance, in which they are gradually carried back to the stomach as the style is thrust down, being no doubt partly digested meanwhile by the amylolytic style enzyme. Owing to the finely divided nature of the food entering the stomach, the delicate style performs no triturating function; its chief mechanical role is to promote the constant circulation of the stomach contents in the vicinity of the ciliary sorting area.

The sorting area (C.ST.A.) occupies all the left aspect of the stomach. It forms an extensive series of ciliated ridges and furrows, at first very narrow, and converging anteriorly to form about 12 wider folds which terminate abruptly at the opening of the intestine. The sorting surface is further increased by a long S-shaped fold (F.) of the wall of the anterior chamber, extending from the end of the typhlosole to the gastric shield. This flap incompletely separates the sorting area from the rotating style head. Its left aspect is thrown into a set of narrow folds, passing obliquely forwards to converge upon the intestinal opening. The grooves between the sorting ridges maintain strong ciliary currents towards the intestine, by which coarser particles are eliminated before passage of the stomach contents to the digestive diverticula. The ridges are also covered, especially the wider folds towards the intestine, by transversely beating cilia, which together with the rotating style, keep finer particles in circulation, for transfer to the diverticula.

The left digestive diverticulum opens by a narrow aperture from the posterior chamber of the stomach, which is lined behind the gastric shield with non-cuticulate ciliated epithelium. Particles are carried to the diverticulum by a posterior-directed ciliary current. Material returned from the digestive gland goes direct to the intestine, along an extension of the sorting area, which passes obliquely across the stomach, below the gastric shield. This ciliated region is separated from the cuticulate region by a narrow rim-like fold. Fine ciliary currents beat into the mouths of the diverticula, carrying the finest divided particles to the digestive epithelium, which forms a very extensive ingesting surface.

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The Digestive Gland

The digestive lobules possess both digestive and excretory cells. The digestive cells (Fig. 13, DI.C.) are tall (100μ) and columnar, with basal nuclei; their cytoplasm is crowded with spherules, formed of greenish-yellow particles clumped together, in little boluses, 7μ across. From time to time these are egested from the cells and returned to the stomach. As in some other prosobranchs, the free cell surface is probably sparsely ciliated in life, but cilia are difficult to identify in sections, and the cell borders take on a convex pseudopodial appearance. Excretory cells (EXC.C.), presumably a means of rejecting absorbed chlorophyllous pigments, occur much less frequently, interspersed with the digestive cells. They are broad-based and pyramidal with brown-staining contents, and usually contain each a single brownish-black spherule, 25μ or more across. These spherules appear with the digestive cell contents in the egested material passed into the stomach. Faecal material is derived from two sources, first from the empty diatom frustules which are never found within the diverticula and apparently after preliminary digestion of their contents are carried directly to the proximal intestine, secondly from the egested particles from the two types of digestive gland cell. As in Struthiolaria, but contrary to the suggestion of Mansour (1946) in certain lamellibranchs, the greenish digestive cell particles are not enzymatic but represent waste products of digestion. In Vermicularia they were actually observed in the living stomach to pass at once by ciliary currents from the anterior diverticulum to the proximal intestine.

Intestine

The first portion of the intestine (Fig. 9, P.INT.) which remains open to the style caecum—is a spacious tube, lined with extremely low (12μ) ciliated cells, by which faecal matter is carried to the middle intestine (Fig. 10, M.INT.). This is a narrow region (0.5 mm. diameter) which describes two short loops within the lumen of the renal sac. It then emerges to a superficial position and widens into the rectum (Fig. 10, RM.) which runs along the right pallial wall to the anal papilla. The epithelium of the middle intestine is uniformly tall, with strong cilia, and mucous cells. There is a narrow coat of circular muscle, and it is here that the individual faecal pellets are constricted off by peristaltic action from a continuous mucus string. Each pellet is 3μ long, narrowly ovoid, and widely distends the middle intestine. It is rotated forward by ciliary action assisted by peristalsis, which moulds the loose detritus and firmly compacts it with mucus. The pellet finally consists of an inner core formed almost entirely of empty diatom frustules and digestive gland egesta, surrounded by an outer pellicle, 50μ wide, of clearer mucus. Passing one by one from the middle intestine into the rectum, the pellets are arranged spirally round the wall, to lie in three longitudinal rows. Each is part surrounded by the indented wall of the rectum, and its formation is completed by further ciliary rotation as it passes forward. The anal region is narrower and more muscular, and its cilia especially long and swift-beating. A mass of faecal pellets is from time to time discharged, enclosed in clear mucus from the rectum, and is at once swept away by the exhalant pallial current.

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III. Reproductive System

Serpulorbis, like the rest of the vermetid genera, is incubatory. The eggs are retained in capsules, attached to the inner surface of the shell, along the median pallial slit of the female. From 3 to 5 capsules are generally produced at one time, each containing about 10 eggs, 0.25 mm in diameter. The free-swimming stage is entirely lost, and the velum of the embryo much reduced in size. On emerging from the capsule membrane, the embryo creeps out of the aperture of the female shell, and is able for a limited time to crawl about, before attaching itself to the substratum and developing the uncoiled adult shell. The male is aphallic, and the sperms are shed from the vas deferens directly into the mantle cavity and carried out by the exhalant current. The female is thus fertilized by the entry of current-borne sperms with the inhalant pallial current. The pallial genital duct is for most of its length widely open for the entry of sperms from the pallial cavity. A similar case of current fertilization with an unclosed genital duct is described by Fretter (1946) in Turritella in which the capsule gland and the albumen gland are both widely open along the ventral surface. The pallial genital duct primitively originated as an unclosed groove, and examples of incomplete closure with current fertilization are to be found also in the Cerithiidae (Zeacumantus, unpublished observation) associated with well-protected pallial apertures in silty habitats, as well as in the sessile vermetids. These cases may have arisen by the re-acquisition of a primitive condition; in the Strombacea on the other hand (Morton, 1949b) copulation occurs, and the unclosed capsule gland is to be attributed to a primitive survival.

The female genital ducts of Serpulorbis zelandicus are illustrated in Figs. 10–12. The narrow ovarian duct (50μ) lined with ciliated epithelium is capable of great distention with the passage of eggs. It opens through the right wall of the albumen gland, which forms the first portion of the pallial oviduct. The lumen of the albumen gland (Fig. 11, ALB.G.) is curved and slit-shaped, closed along the lower edge, and lined by a single row of tall, columnar cells (30μ) filled with secretion spherules staining light-brown with Van Giesen's. These cells secrete the albumen coat surrounding the eggs after fertilization. The wall of the albumen gland is strongly ciliated, and sperms are carried backward from the capsule gland along the ventral side of the lumen to the receptaculum. This is represented by a small spherical pouch, 1.5mm.–2mm. in diameter, attached dorsally to the posterior end of the albumen gland. The lining is of short, cubical epithelium, and a dense row of haematoxylin-stained sperm heads is generally attached to the surface of the cells. It may be suspected that a chemotactic stimulus is responsible for the invariable aggregation of current-borne sperms in this part of the genital duct.

The capsule gland (Figs. 10, 12, CPS.) is much larger than the albumen gland. It forms a pouch-shaped organ, 9–10 mm. long, pale yellow in colour, open along its whole ventral surface into the pallial cavity. The epithelial cells are of two sorts, narrow ciliated cells, with a short ciliary coat (50μ) and dark rod-shaped nuclei, and columnar gland cells, 50μ tall, whose secretion stains lightly with haematoxylin. A single egg capsule occupies the whole lumen of the gland, and the

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group of eggs, with its secreted envelope, is then attached directly to the shell, the first formed capsule most anteriorly. As the capsule is attached, the capsule gland cilia rotate it to twist off a short stalk, and albumen coat is also twisted into a chalaziform strand. The embryos move about to some degree within the capsules, and the membrane is finally ruptured by the sharp edge of the embryonic operculum, which has attained its maximum size at the time of liberation.

Discussion

The foregoing description of Serpulorbis zelandicus affords a comparison with “Vermetus” novae-hollandiae, an exclusively ciliary feeding species (Yonge, 1932) and Serpulorbis gigas which has been shown (Boettger, supr. cit.) to rely wholly upon mucus traps for food collecting. Yonge was inclined to separate fairly widely the two latter forms and suggested that the Vermetidae were probably divisible into separate groups on the basis of their widely differing feeding mechanisms, stating (1932) “the taxonomy of the Vermetidae clearly requires revision in the light of these results.” Serpulorbis zelandicus, is now found to show a condition intermediate in most respects between novae-hollandiae and gigas, suggesting that ciliary and mucus trap feeding represent merely extremes of a single evolutionary series. Without at present opening the question whether all of the forms at present included in the Vermetidae are closely related, the evidence is strong that the three forms above discussed should be regarded as belonging to a single natural group. In each case the adaptation of the foot is fundamentally the same, and the detailed structure of the pedal gland and tentacles is closely similar. In the alimentary canal likewise, novae-hollandiae as described by Yonge (1932) shows a well recognizable resemblance to sipho; the digestive system of gigas, though the description of Yonge and Iles (supr. cit.) is merely a short outline, appears to conform to the same plan. In each species the pharynx is relatively prominent and bulbous, and the salivary glands minute with short ducts. The anterior portion of the oesophagus is dilated, being crop-like in novae-hollandiae, cylindrical and rather less wide in gigas and zelandicus. The structure of the stomach and style caecum, and the disposition and relative size of the digestive diverticula also agree in novae-hollandiae and gigas. Between gigas and zelandicus there is especially close agreement in the radula, the figure of Troschel (Das Gebiss der Schnecken) for gigas being almost identical with Text-fig. 1 above, affording strong support to the return of the New Zealand shells to Serpulorbis. The chief feature in which zelandicus differs from gigas and resembles novae-hollandiae is in the retention of the ciliary feeding habit, and the consequent high development of the ctenidium and food tract. Bivonia, represented by triqueter, described by Lacaze-Duthiers, is also typical of the present group in structure of the foot, and in the alimentary and reproductive systems. The structure of the genital ducts is incompletely known, but there is evidence that zelandicus is here again typical of the whole of the present group.

“Vermetus” novae-hollandiae stands somewhat apart from the other forms above by its extremely large size, and the straight cylindrical shell. The ciliary feeding condition in novae-hollandiae is no doubt a more primitive stage of vermetid evolution, and it should

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be noted that this species retains throughout life the large slightly concave operculum, with the same relative proportions as in the embryo of zelandicus. The gill filaments are strongly developed and powerfully ciliated, though from their triangular shape, as well as from the lack of an endostyle and of a specialized food groove, it would appear that members of this group never become so completely adapted for ciliary feeding as some of the other style-bearing prosobranchs. Moreover, although the pedal gland in novae-hollandiae is much less well-developed than in either zelandicus or gigas it is yet of relatively considerable size. It may be suggested that this structure was not originally developed in the Vermetidae solely for the production of mucus for ciliary feeding, for which requirement the food tract mucus supply would seem sufficient. Although novae-hollandiae—occurring as stressed by Yonge in a wave-beaten environment—does not resort at all to mucus trap feeding—it is likely that most other species of this group may—like zelandicus—possess to some extent the ability to form mucus traps. Bivonia triqueter is a mucus trap feeder according to Yonge (1932), while on the north American Pacific coast, Spiroglyphus lituellus and Aletes squamigerus—as figured by Johnson and Snook (1935) have the same structure of head, foot, pedal tentacles and food tract as described above. Ricketts and Calvin (1948) briefly state the last-named species feeds by the formation of mucus strings. “Vermetus” maximus and “Vermetus” giganteus are stated by Yonge and Iles (supr. cit.) to possess the more primitive ciliary feeding condition of novae-hollandiae, though their exact relationship to the above species has yet to be ascertained.

It is probable—as suggested by Yonge and Iles (1939)—that the presence of operculum and a powerful ctenidial current tends to impede the employment of mucus strings. In zelandicus, however, mucus feeding appears to be carried out to a minor extent in the presence of a strong current passing into the pallial cavity. Pari passu with the loss of the operculum and the enlargement of the pedal gland, the ctenidium is reduced in size in vermetids, reaching its smallest proportions in gigas, which is probably mainly dependent on pallial respiration. A parallel evolutionary trend seems to be the reduction of the crystalline style. According to Yonge's figure the style is large and stout in novae-hollandiae; in zelandicus it is much more slender, and very delicate and impermanent. Its presence in gigas was first denied by Yonge (1932), but examination of living material (Yonge and Iles, 1939) later revealed a style and a small gastric shield “relatively much smaller than in ‘Vermetus’ novae-hollandiae.” It would be interesting to find whether the style is ever actually lost in mucus trap feeders; the ingestion of predominantly animal material such as small isopods captured by gigas (Boettger, 1930) would greatly diminish the need of a style. Yonge later found small gastropod shells in the stomach of gigas, and it seems well established that the mucus feeders are collectors of zooplankton. The presence or absence of a crystalline style is primarily a physiological character, which is probably not of major taxonomic importance—witness its separate appearance in gastropods in such groups as the prosobranchs and the thecostomatous pteropods.

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Summary

The taxonomy of the New Zealand vermetids of the genus Serpulorbis is discussed. The shell, animal and dentition of S. zelandicus (Q. & G.) are described, and a new species, S. aotearoicus, is proposed. Novastoa zelandica becomes Novastoa lamellosa (Hutton). A full description is given of the external features, pallial organs and mode of ciliary feeding. Resemblances to “Vermetus” novae-hollandiae are indicated. In addition, S. zelandicus is shown to possess a large pedal mucous gland, and to feed to some extent by the supplementary mode of forming mucus strings. The species is regarded as illustrating the transition from exclusive ciliary feeding in “Vermetus” novae hollandiae to the complete development of mucus trap feeding in Serpulorbis gigas. The morphology and histology of the alimentary canal of Serpulorbis zelandicus is described, with an account of the ciliary currents and mode of function of the stomach and crystalline style caecum. The female reproductive system shows adaptations for current fertilization and for incubation of the eggs within the shell of the female. A comparative discussion follows, on the position of Serpulorbis zelandicus with relation to other members of the Vermetidae. Yonge's view that the ciliary feeders and the mucus trap feeders form two distinct groups is not upheld. A continuous evolutionary trend is shown from the more primitive ciliary feeding condition to mucus feeding, with reduction of the ctenidium and enlargement of the pedal gland. The genera Bivonia, Aletes, Petaloconchus, and Spiroglyphus are associated with the present series.

References to Literature

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Cranwell, Lucy M., and Moore, Lucy B., 1938. Intertidal Communities of the Poor Knights Islands, New Zealand. Trans. Roy. Soc. N.Z., 67, 375–407, pls. 53, 54.

Finlay, H. J., 1927. A Further Commentary on New Zealand Molluscan Systematics. Trans. N.Z. Inst., 57, 320–485, pls. 18–23.

——, H. J., 1928. The Recent Mollusca of the Chatham Islands. Trans. N.Z. Inst., 59, 232–286, pls. 38–43.

Fretter, Vera, 1946. The Genital Ducts of Theodoxus, Lamellaria and Trivia, and a Discussion on their Evolution in the Prosobranchs. Journ. Mar. Biol. Assoc. U.K., 26, 312–349.

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——, A., 1938b. On the Alimentary Canal in the Style-bearing Prosobranchs. Proc. Zool. Soc. Lond. (B), 107, 75–112.

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Morton, J. E., 1951a. The Structure and Adaptations of the New Zealand Vermetidae. Part II. The genera Stephopoma and Pyxipoma. Trans. Roy. Soc. N.Z., vol. 79, 20–42.

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——, C. M., 1938. Evolution of Ciliary Feeding in the Prosobranchia, with an Account of Feeding in Capulus ungaricus. Journ. Mar. Biol. Assoc. U.K., 22, 453–468.

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