The Structure and Adaptations of the New Zealand Vermetidae
Part II. The Genera Stephopoma and Pyxipoma
[Read before the Auckland Institute May 17, 1949; received by Editor, February 1, 1950.]
|I.||Stephopoma Roseum Systematics The Pallial Organs and Foot: Feeding and Cleansing Mechanisms. The Alimentary Canal.|
|III.||Reproductive System Stephopoma and Pyxipoma.|
|IV.||Discussion. Synonymic List. Acknowledgment. Summary. References to Literature.|
The feeding mechanism and general structure of Serpulorbis zealandicus (Q. & G.) and S. aotearoicus Morton, the largest and most widespread of the New Zealand vermetids, have recently been described (1951a) by the present writer, and the relationships of these species discussed, so far as our present knowledge of the Vermetidae extends. No previous work has been carried out on the living animal of the genus Stephopoma, of which the type is the neozelanic roseum. An examination of the structure and mode of feeding of this species now makes it clear that Stephopoma differs widely from the vermetids previously investigated. Its closest relationships are with the siliquariids, and in the present paper it is proposed to deal also with the species Pyxipoma weldii. The remaining member of the New Zealand Vermetidae, Novastoa lamellosa, belongs to a group of which the living animals are still unknown, and will form the subject of a separate account. A revised list of the neozelanic vermetids, with synonymy, is appended to this account.
I. Stephopoma roseum (Quoy and Gaimard).
(Equals Stephopoma nucleogranosum Verco, Suter's Manual;
not of Verco.)
Suter has left the systematics of this species in some confusion. The Manual recognises two species of Stephopoma in New Zealand—namely, roseum (Quoy and Gaimard) and nucleogranosum Verco, which are distinguished by Suter thus:—
Whorls not carinated, protoconch smooth; bristles of operculum simple, multifid at base only roseum
Whorls carinated, protoconch minutely granular, bristles of operculum multifid nucleogranosum
Specimens of “roseum” in Suter's sense have eluded discovery. There appears to exist in collections and in the field a single species only of Stephopoma, common in various localities in the Hauraki
Gulf and northern part of the North Island. The type locality of roseum is “Thames River in considerable depth.” Suter referred the common Hauraki Gulf shells to nucleogranosum, the type of which is South Australian; he differentiated this material from roseum apparently on the grounds of Quoy and Gaimard's original description of the latter species, which is translated directly into the Manual. That Suter was not personally acquainted with a species with “protoconch smooth; bristles of operculum simple” is suggested by the single specimen labelled roseum in his collection—“3726 Stephopoma roseum, Q. and G. 4 f. Rangitoto Channel.” This specimen is the basis of the sole record of roseum, apart from the type locality, in the Manual. The shell is worn and imperfect with the apex eroded. The whorls are smooth shouldered with a “Cyclostoma-like appearance,” but the specimen cannot be satisfactorily separated from the common Hauraki Gulf species, in which the shape of the whorls, particularly in beach-worn shells, is highly variable.
It is at least unlikely that the species described by Quoy and Gaimard as Vermetus roseus should not have turned up during the whole of the following century, and on reading the original account of this species, we may safely draw the conclusion that roseum and the neozelanic shells referred by Suter to nucleogranosum are one and the same species. Particularly is this view supported by the reference to “Thames River” as the type locality of roseum. The shells referred to nucleogranosum are common in the Hauraki Gulf, from which opens the Firth of Thames, and no other species of Stephopoma is upon record from this locality. In addition, the details of the animal given by Quoy and Gaimard are quite recognisably applicable to the Auckland species. Nor are the shell features taken by Suter from the original description sufficient to mark off two New Zealand species of Stephopoma, having regard to the different convention of description and illustration of a century ago. The rounded, non-carinate whorls might well belong to a specimen of the extremely variable Auckland species, in which, especially in the later whorls, the shoulder is often obsolete or wanting. Suter's statement in his key to the genus “protoconch smooth” is his only interpolation into Quoy and Gaimard's description, and is certainly not warranted from the material available to him. Quoy and Gaimard do not specifically mention the nucleus; though, as was long ago suggested by Morch, their reference to minute holes, as of some parasitic animal, may be due to a mistaken microscopic interpretation of the pustulations of the embryo shell, referred to below. A similar imperfection in microscopical technique was probably responsible for Quoy and Gaimard's figure of the opercular bristle, which is poor and oversimplified. The bristle represented is unlike that of any other Stephopoma, and cannot be safely relied upon. The colour description of roseum applies well to the Auckland species; unencrusted living shells are frequently flushed with pink or pinkish brown.
Thus the New Zealand nucleogranosum becomes Stephopoma roseum as the sole neozelanic representative of the genus. Verco's South Australian species, however, remains valid, by virtue of slight differences from the New Zealand shells, which were predicted by Finlay (1927). The writer is greatly indebted to Mr. B. C. Cotton,
of the South Australian Museum, for shell and operculum material of Stephopoma nucleogranosum. Roseum differs from nucleogranosum in the sculpture of the nucleus, which is ornamented with closely spaced, regularly arranged tubercles or pustules, relatively larger than the “numerous minute granules” of Verco's species. The pustules are arranged in rows along the growth lines, giving the nucleus a tessellated appearance back to the apical half whorl, which is a tiny smooth bulb. A row of 30–40 well-developed pustules runs round the periphery—a feature which Verco states to be quite wanting in the nucleogranosum. The aperture of the embryo shell is, as in nucleogranosum, trumpet-shaped, its rim projecting narrowly at the inception of the adult shell, which is thick and smooth, with fine accremental striae. Verco's figure of the opercular bristle is not quite like that of the South Australian specimen examined by the writer. In Milford roseum the opercular bristle differs only from that of nucleogranosum in the origin of asymmetrical lateral branches, which in roseum (Text Fig. 1) spring straight from the main shaft, but in nucleogranosum arise by the division of a single branch axis. This character may, however, be somewhat variable, and is probably not of constant taxonomic value.
Text Fig. 1—Stephopoma roseum Milford. A single opercular bristle from the outermost
annulus. Half the length of the long mediun seta has been left out. X
Dentition: Unknown in nucleogranosum; figured for roseum (Fig. 23). The central tooth has a short median cusp, flanked by four minute denticles on either side. The laterals are long and slender, strongly hooked antero-medially, the marginals curved and falciform, without serrations.
Localities for Stephopoma roseum: Thames River (type) Q. and G.; Rangitoto Channel in about 5 fathoms, Takapuna Reef, Bay of Islands (H.S.); Awanui Bay, North Auckland, embryos dredged in 12 fathoms (Finlay Collection, per W. la Roche); Milford Reef, under boulders at low spring tide; Otata Island, Noises Group, on vertical rock faces at low spring tide (J.E.M.).
The close resemblance of nucleogranosum to roseum is worthy of remark; when we consider also the fact that the New Zealand Pyxipoma is conspecific with the Australian P. weldii, it is evident that the present group of vermetids—unlike Serpulorbis—does not well exemplify the usually clear-cut disparity between Australian and
neozelanic molluscan faunas. It is doubtful whether the genus Lilax, as proposed by Finlay (1927) is now really required for the Australian shell.
Stephopoma roseum is a typically low-tidal species, accessible only at low water spring tides. At Otata Island it is present as the dominant sessile member of a low-tidal animal association, on the vertical sides of rocky channels through which there is a strong wave surge. Here it forms a zone two or three inches in vertical height, immediately below a band of serpulid worms, and plentifully admixed with pink encrusting corallines. Serpulorbis aotearoicus is present in sparser patches immediately below. At Milford and Takapuna, Stephopoma roseum is a co-dominant sessile animal on the undersides of rocks, together with the barnacle Elminius modestus, sponges and hydroids, and the ascidians Corella eumyota and Didemnum candidum, and a rich collection of non-sessile associated animals. Unlike Serpulorbis, Stephopoma is very tolerant of sheltered backwaters, where the exposed rock surfaces are covered with silty Corallina officinalis, and there is a fine sandy deposit beneath the stones. Clusters of Stephopoma tubes are frequently aggregated at the sides of the boulders near the lower edge of the Corallina fringe, and are here densely covered with a green nulliporite or pink basal Corallina. Shells underneath the rocks in contact with the substratum are translucent white, or pinkish tinted. The species is evidently limited at its upper margin by the exposure factor. It tolerates wave surge as well as Serpulorbis, while in addition, having a protective operculum and more efficient cleansing mechanism, it thrives well in silty locations.
The Pallial Organs and Foot: Feeding and Cleansing Mechanisms.
When removed from the shell Stephopoma roseum is seen to be spirally coiled; the visceral mass is never irregularly vermiform as in Serpulorbis. The foot forms a stout plug-shaped column, surmounted by a very prominent operculum (Fig. 1 OP.). This is a thick chitinous disc, multiannular, and with each annulus fringed at its free edge with long multifid setae, figured in Text Fig. 1. The glandular regions of the foot are white in colour, forming a conspicuous area in front of the mouth. There are no pedal tentacles, but the front edge of the foot is produced into a median vertical process in front of the mouth, the pre-oral appendage discussed below. The rest of the foot and the exposed parts of the head are dark black or grey, and the trunk and pallial region whitish. The proboscis (Fig. 1 RO.) is short and bluntly rounded, jet black in colour, with the mouth a vertical slit at its tip. A pair of minute tubercles (TNT.) represents the cephalic tentacles, with small eyes at their outer bases. The pallial cavity forms a spacious chamber, and its contained organs may be conveniently considered in discussing the feeding process.
Stephopoma is a ciliary feeder on diatoms collected from the pallial current by the ctenidial filaments. The mechanism of feeding conforms to the fundamental type made familiar by the accounts of Yonge (1938) and other writers on ciliary feeding prosobranchs. There are, however, numerous differences in detail, and while most closely resembling Crepidula, Stephopoma is in some respects more
specialized than any of the previously known forms. The gill filaments (G.FIL.) are long, flexible, and rod-shaped. They are freely movable, and attached to the mantle for a short distance only, along the extreme acial side of the gill. They reach across the mantle cavity to the right to form a sort of temporary septum between ventral inhalant and dorsal exhalant chambers; (Text Fig. 2); but the whole gill is much less rigid than in Crepidula (Orton, 1912), where the filaments form a set of stiff rods. In Stephopoma there is a noticeable power of independent muscular movement. The separate filaments frequently bend and curve slightly, especially near the tips where the skeletal rods are vestigial and the intrinsic longitudinal muscles are well represented. The gill does not quite cover the hypobranchial gland (Fig. 5 HY. GL.), which is well developed, covering the right side of the mantle as a series of opaque white, transverse folds. The rectum opens by the anal papilla some distance behind the mantle margin. Immediately below the rectum is the ciliated genital furrow in the male—terminating simply, without a penis, and in the female, the glandular pallial genital duct. The renal organ opens on the mantle roof by a narrow slit, mesially to the rectum, rather far back.
Text Fig. 2—Stephopoma roseum. Diagrammatic view of the foot and head region,
part expanded and viewed from above, showing the course of the ciliary currents.
The setose crown of the operculum has been omitted. X. AP, pre-oral process;
CT, gill filament; EN, endostyle; EXH, exhalant region of the pallial cavity;
F.G, food groove; GL.F, glandular sole of foot; INH, inhalant region of pallial
cavity; OP, operculum (simplified); PA, mantle; REJ, lateral rejectory tract of
foot; RO, proboscis; SI, siphonal tubercle.
The mantle margin is entire, without inhalant siphon, and densely speckled with black. There is a single row of blunt papillae, repre-
senting a fringe of very short pallial tentacles. A powerful current is drawn into the pallial cavity on the left by the beating of the cilia of the gill. Such large particles as acutally alight on the pallial margin are quickly removed by fine, outward beating cilia. The remainder travel back along the line of the osphradium, being drawn obliquely to the right on to the frontal surface of the gill. The osphradium has the structure usual in primitive mesogastropods—a ridge overlapping on either side a line of sensory cells, and finely ciliated along its edges, which are black-pigmented. The length of the sensory zone is increased by close-spaced meanders and the osphradium extends the whole length of the gill. It is well situated for the important role suggested by Hulbert and Yonge (1937)—of determining the amount of sediment entering the pallial cavity. Between the asphradium and the gill axis lies a narrow glandular tract, the endostyle, serving for the supply of mucus in which are entangled the particles reaching the gill in the inhalent current. In Serpulorbis (Morton, 1951a) the endostyle is undeveloped; but its presence in Stephopoma is important owing to the virtual disappearance of the glandular areas on the gill filaments. The endostylar epithelium is of the type described by the writer in Struthiolaria (1951) and present also in Turritella: cigar-shaped glandular cells alternate regularly with tall, narrow, ciliated cells (50–60μ) keeping up a constant transverse beat towards the gill. Stephopoma has not—like Crepidula (Orton, 1914)—developed separate tracts of ciliated and glandular cells, after the manner of the unrelated pharyngeal endostyle of the lowest chordates.
The gill filaments possess the usual ciliary tracts (Fig. 6). The frontal cilia (FR.C.) are short but fast-beating, extending along the ventral aspect of each filament, facing the pallial cavity floor, and carrying mucus-bound particles direct to the edge of the gill. The lateral cilia (LAT.C.) are extremely long (40μ)—as in Crepidula, a good deal taller than the width of the filament itself. They lash dorsally between the filaments, and effectively strain off particles which are retained on the frontal side. Their metachronal beat is very distinctive, the rapid wave passing in opposite directions along the anterior and posterior sides of each filament. There is a tract of smaller and weaker abfrontal cilia (AB.C.) along the dorsal aspect of each filament, beating—like the frontals—towards the apex. The tip of each filament is equipped with a tuft of very tall (50μ–60μ) generally motionless apical cilia (AP.C.) whose functional role is discussed below. In transverse section (Fig. 6, a, b, c; Fig. 14) the gill filament shows a less specialised condition than in Crepidula: the slender skeletal rods remain—as primitively—close to the frontal side, and the respiratory area of thin cuboidal epithelium—though much reduced—is still present. The gland cells of the respiratory area are almost lost, and contribute but little of the mucus for the collection of particles. As well as from the endostyle, mucus is probably received from the hypobranchial gland in considerable amount. At frequent intervals the row of filaments is flexed upwards against the roof of the mantle cavity in close contact with the folds of the gland.
During feeding the filaments normally bend downwards towards the floor of the mantle cavity. Their tips are closely pressed together into the food groove, especially anteriorly where several filaments
generally overlap in a cluster (Fig. 4, G.FIL.). The food groove is a shallow gutter along the surface of the trunk. It is bounded on the right by a thin fold of ciliated and glandular epithelium. Passing towards the head it narrows considerably, and the bounding fold enlarges, being capable of much muscular movement. Its edge is often crenellated or frilled; the inner surface is uniformly ciliated. The fold curves, runs forward to terminate level with the mouth, just below the right tentacle (F.G.). The food groove cilia keep up a constant current passing forward all particles deposited by the gill filaments. The tip of each filament is rather expanded and bulbous, uniformly covered upon the frontal aspect with short terminal cilia (T.C.). These have a fast beat forwards across the surface of the filament, and when in contact with the food groove contents play an important role in moving food particles. The bristle-like apical cilia are kept perfectly motionless when the filaments are removed and examined. They are most typically observed in action as described below, when the anterior portion of the gill is protruded from the pallial cavity. But they probably also have some part in the forward movement of material in the food groove. Anteriorly the contents of the food groove are continually rotated by the lining cilia, and the terminal fold imparts a strong kneading action by muscular contractions. Finally a thick mucous cord, rotating in clockwise direction, is protruded from the opening of the groove for ingestion at the mouth (Fig. 4). Only the finest of particles, such as diatoms and flagellates, find their way into the food groove. Carmine or other foreign material entering the pallial cavity is quickly collected by the gill and swept away without entry to the groove.
The glandular portions of the foot in Stephopoma (Fig. 1, Text Fig. 2) are adapted for collecting and rejecting waste particles alighting near the head or carried out of the pallial cavity. There is no trace of the specialised pedal mucus gland described in “Vermetus” and Serpulorbis, nor of ciliated pedal tentacles. The gill alone is employed in food-collecting, and mucus traps are never put out from the foot. The original plantar surface of the foot now forms a convex pad (G.FT.) white in colour, situated between the margin of the operculum in front, and the mouth behind. It is finely ciliated and densely supplied with mucus glands. The anterior portion of the foot is narrowly constricted from the sole, forming the tall, papilliform, pre-oral appendage which stands up vertically in front of the mouth. Running forwards towards the operculum, around the margins of the sole, are two wide, shallow grooves (REJ.). They converge posteriorly at the base of the pre-oral appendage, and are greyish-black pigmented, bounded by muscular, somewhat crenellated edges. They serve as rejection tracts for unwanted particles from the neighbourhood of the mouth and from the debouchment of the food groove. Waste material is compacted into a rounded bolus, enclosed in mucus, and is rejected from the foot at the mid-line. A load of rejecta from time to time accumulates in the bristles of the operculum, and is ultimately swept away by the sudden retraction of the food, or the passage of the exhalant current.
The pre-oral appendage (AP.) is white in colour, very muscular and labile. It assumes a variety of shapes, being generally spatuli-
Fig. 1—Stephopoma roseum. The head and foot region of the fully expanded animal,
showing the ctenidial sweeping fringe and the course of the feeding and rejectory
Fig. 2—Pyxipoma weldii. The complete animal removed from
the shell and viewed from the right side.
Fig. 3—Pyxipoma weldii. The base of the operculum, viewed from below.
Fig. 4—Stephopoma roseum. Head region drawn from life, showing the action of the
radula in detaching a food bolus from the mucus cord in the food groove.
Fig. 5—Stephopoma roseum. Semi-diagrammatic transverse section through the trunk
and pallial cavity in front of the anus. Portions of two successive gill filaments
here appear in the same section.
Fig. 6—Stephopoma roseum. Gill filaments. a, b, c. Transverse sections of filaments,
near the base (a), at mid length (b), and at the apex (c). d, e. Ventral view of
tip of filament, showing apical cilia inert (d) and in action during food collection
or rejection of particles.
AB.C, abfrontal cilia; AP, pre-oral appendage; AP.C, apical cilia; B.P, opening of
brood pouch; CM, style caecum; CT, ctenidium; END, endostyle; E.SI, exhalant
siphonal appendage; F.G, food groove; FR.C, frontal cilia; F.BOL, food bolus;
G.FIL, gill filament; G.FT, glandular region of the foot; HY.GL, hypobranchial
gland; INT, middle region of intestine; K, renal organ; LAT.C, lateral cilia;
M.BOL, mucus bolus of material rejected from foot; OE, oesophagus; OP, operculum;
OS, osphradium; R, rectum; RA, radula; REJ, rejectory tract of foot;
RO, proboscis; SK.R, skeletal rod; SL, pallial slit; ST, stomach; T.C, terminal
cilia; TNT, cephalic tentacles.
Fig. 7—Female genital ducts and intestine viewed from the right side.
AL, albumen gland; CP, capsule gland; K, renal organ; MI, middle intestine;
OV, ovarian duct; RM, rectum.
Fig. 8—Stomach and Crystalline Style Caecum dissected from the dorsal aspect, showing
the course of the ciliary currents.
Fig. 9—Photomicrograph of transverse section of oesophagus, shortly before its opening
into the stomach.
Fig. 10—Photomicrograph of transverse section of the head, showing pharyngeal bulb,
eye (on left) and tips of anteriormost gill filaments (on right).
Fig. 11—Transverse section of style caecum and proximal portion of intestine.
Fig. 12—Photomicrograph of transverse section of a single tubule of the digestive gland.
Fig. 13—Photomicrograph of transverse section of stomach, through gastric shield,
sorting area and digestive diverticulum.
Fig. 14—Photomicrograph of gill filaments in transverse section.
CM, style caecum; C.S., ciliary sorting area; DT, dorsal typhlo [ unclear: ] ole; DV, digestive
diverticulum; EY, eye; F, crescentic fold referred to in text; G.FIL, gill filament;
I.GR. groove of proximal intestine; M.I., middle intestine; OD, odontophore;
OE, oesophagus; SH, gastric shield; ST, crystalline style; VT, ventral typhlosole.
Figs. 15, 16, 17—Pyxipoma weldii. Embryo shell removed from brood pouch, showing
operculum closing aperture (16) and part open (17).
Fig. 18—Stephopoma nucleogranosum. South Australia. Nuclear portion of mature
Fig. 19—Stephopoma roseum. Embryo shells.
Fig. 20—Stephopoma roseum. Milford Reef, Rangitoto Channel. Embryo enclosed in
capsule membrane, removed from pallial cavity.
Fig. 21—Stephopoma roseum. Milford Reef, Rangitoto Channel, Embryo at creeping
stage, after emergence from capsule membrane, removed from throat of adult
shell. CT, cephalic tentacle; F. foot; F.G, terminal portion of food groove;
GF, gill filaments; MO, mouth; OP, operculum; PA, mantle; PG, anterior pedal
Fig. 22—Pyxipoma weldii. A single row of radular teeth, marginals omitted on right.
Fig. 23—Stephopoma roseum. A single row of radular teeth.
form with the edges at times incurved to form a sort of temporary tube. Its function was difficult to determine exactly, but it appears to be employed in connection with both feeding and waste rejection. It is frequently curved to the left towards the opening of the food groove, or held in front of the mouth as a spoon-like lobe, perhaps assisting the prehension of the mucus cord by the radula. In the related Pyxipoma (Fig. 2) where the food groove termination is curved round to the mid-line, and the bolus carried straight to the mouth, the pre-oral appendage is not developed. Material from the food groove that is not ingested at the mouth, falls immediately on the lateral rejection tract of the right side of the foot. It is thence quickly carried forward to the base of the operculum, and this is also the case with large foreign particles extruded from the mantle cavity. When carmine or carborundum is introduced by the inhalant current, the gill is temporarily lifted free of the food groove. The particles are rapidly carried across the gill by the frontal cilia, embedded in mucus from the endostyle. The now activated apical cilia carry the mucus string forward and it is immediately swept out of the pallial cavity with the exhalant current. The hypobranchial gland appears to play only a minor role in the rapid elimination of waste. The pre-oral appendage assists in the rejection of smaller particles, such as finer grades of carborundum. Particles are carried by cilia up the middle line of the posterior side of the appendage, and round the tip, which is temporarily indented to allow material to pass over the top. They are now carried downwards along the anterior aspect, diverging to right or left sides, to be carried across the glandular cushion of the foot, and finally swept forward along the lateral rejectory tracts.
Apparently the animal may feed normally for a considerable time with the foot part retracted into the shell. The operculum does not then entirely close the shell, but rests with its bristles pressed against the shell edge; the inhalant current thus passes through a sieve-like mesh of bristles, which may thus have a function similar to the pallial tentacles of Turritella, in guarding against the entry of excessively large particles. The operculum is slightly smaller in diameter than the shell tube, and the animal may retreat within the shell, with the flexible bristles bent forwards to allow backward retraction of the foot.
On the right side of the foot, to the right of the food groove opening, is a structure corresponding to the exhalant siphonal tubercle of Turritella. It forms a short, stout, triangular papilla, backwardly pointed, and often slightly curved (E. SI.). It is apparently not ciliated, though traversed by a shallow groove running from base to tip. From its base a fold of integument runs back to the bounding fold of the food groove, along which the sperm groove runs in the male. Though the papilla is present in both sexes, it may well serve to guide the outward current of sperm released by the aphallic male. In addition, especially in the part retracted animal, it curves backward so as almost to enclose a circular opening to the right of the food groove, through which the exhalant current issues from the mantle cavity. The rudiment of the siphonal appendage is present at an early stage in the creeping embryo, together with the food groove fold (Fig.
21, F.G.). At the same stage the anterior edge of the foot is narrow and squarish, very labile and well ciliated. It is already reminiscent of the pre-oral appendage to which it gives rise in the sessile adult.
Stephopoma roseum kept alive in the laboratory was occasionally observed to employ the gill in a second type of food collecting action (see Fig. 1). Although difficult to induce regularly under artificial conditions the details of this process are extremely interesting. When the animal is fully extended, the head and foot project a good distance from the shell, and the anterior part of the gill—about one fourth its total length—is protruded from the pallial cavity by the extension of the mantle skirt. The projecting filaments radiate to form a wide, semi-circular fringe of flexible cirri, which is able to be drawn through the water in a manner somewhat like the sweeping net of a cirripede, though at longer intervals and in a more leisurely manner. At regular intervals the ctenidial fringe is curved back sharply over the rim of the mantle, so that the filament tips are bent down close to the sides of the shell. Almost immediately there follows a rapid recovery sweep, the filaments resuming their original spreading position (Fig. 1). By supplying carmine particles to the projecting portion of the gill, the nature of the ciliary currents can be detected. The mucus secretion from the endostyle is especially copious, and is carried out rapidly to the tips of the filaments by the frontal cilia. The fast-lashing lateral cilia meanwhile temporarily cease beating. This effectively prevents the mucus supply being swept inwards between the filaments and lost. Particles deposited on the gill filaments are swept to the tips, entangled in mucus, which rapidly accumulates as a continuous rope, passing from filament to filament along the frontal side of the gill. It is now that the long, generally inert apical cilia come into play (Fig. 6, e). With a slow uniform beat, they pass the mucus rope around the edge of the fringe towards the head region. The beat of the apical tufts, as illustrated in Fig. 1, is thus backwards in direction in this part of the gill, from the anteriormost to the more posterior filaments, as far as the point where the gill emerges from the mantle cavity. The fate of the collected particles may differ. Large indigestible carmine masses are cast off the edges of the gill on to the rejection tract of the foot, which carries them forward and discards them. Finer particles, however, appear to be deposited actually within the food groove, as they reach those filaments of the gill which still remain dipped into the groove. In this way diatoms and other suspended organisms are added to the food collected by the gill from the inhalant pallial current, as described above. In natural conditions it is probably the stimulus of particles alighting on the gill that induces the abundant secretion of mucus for the sweeping net mode of feeding. Such a method of food collecting is supplementary to the more usual employment of the pallial ciliary currents. It was not possible to determine to what extent relatively the animal relies upon one method or the other, especially as Stephopoma is extremely shy in the aquarium tank, and as a rule remains withdrawn into the shell, or with the aperture about half opened. The development of a ctenidial sweeping net has not previously been recorded in a mollues; such an adaptation is extremely effective in enabling a sessile animal thickly clustered together to
exploit to the full its available feeding area. Stephopoma is thus to be regarded as ecologically equivalent to the sessile cirripedes and the tubicolous serpulid polychaetes.
The Alimentary Canal.
The digestive system of Stephopoma roseum has the general plan typical of a style-bearing prosobranch, most resembling that of Turritella (Graham, 1939) in degree of specialisation. Attention is here devoted chiefly to the considerable differences in detail from Serpulorbis, previously described by the writer (1951a). In the buccal bulb, the jaw plates are reduced to a thin flexible cuticle lining the sides of the mouth. Chitinous mandibular rods are present in a small patch on either side, secreted by underlying columnar cells; they play little part, however, in seizing food material, which function is performed by the sharp, highly erectile teeth of the radula. When the odontopore is protruded the curved marginal teeth and the single-pointed laterals form a set of tiny grappling hooks by which a bolus is detached from the food groove mucus cord, and withdrawn into the mouth. The teeth are subjected to little wear and tear, and the radular caecum, from which the radula is replaced, is very short, terminating immediately behind the pharynx. The odontophoral musculature is slender and reduced, much as in Turritella (personal obs.), and in contrast with the ciliary feeding Crepidula, where the muscles that protrude and retract the odontophore remain unusually large.
The oesophagus (Figs. 5, 8, 9, Oe.) takes its origin in a glandular and ciliated dorsal channel (see Fig. 10) bounded by lateral folds, and forming the roof of the pharynx. A pair of tiny salivary glands open at this point by short ducts, and are composed histologically of mucus cells alone. Immediately behind the pharynx the oesophagus descends vertically, to pass through the nerve ring, after which it turns sharply backwards and passes to the stomach as a uniformly narrow tube (0.15 mm. in diameter). In marked contrast with the dilated crop-like structure in “Vermetus” novae-hollandiae, and the wide anterior region in Serpulorbis zelandicus, the oesophagus in Stephopoma has lost all trace of its primitive division into dorsal glandular and ventral food-conducting portions. The condition is as in the middle and posterior oesophagus of Turritella and Crepidula (Graham, op. cit.). Ciliary cells are uniformly present, with tall cilia (10μ–12μ) and the epithelial folds are about equally developed. Plump mucous goblet cells are prominent, their contents staining rather lightly with haematoxylin. Posteriorly there appear some well-defined longitudinal ridges carrying the oesophageal food string back to the stomach. The oesophagus is here frequently rather sinuous in course, becoming straightened out when the animal is protruded from the shell and the trunk region fully extended.
The stomach (Fig. 8) is a small, triangular sac, opening widely in front to the crystalline style sac, a short, bluntly rounded caecum, 0·75 mm. long in open communication with the intestine along its left side (I.GR.). The anterior region of the stomach, which contains the rotating head of the crystalline style, is lined on the right side with transparent cuticle, continuous below with a small rigid gastric shield (SH.). This forms a triangular shelf of hard cuticle, secreted by a
projecting flange of tall epithelial cells. From the opening of the oesophagus on the left, a wide crescentic ridge (Fig. 8, F.) passes backwards into the narrower posterior portion of the stomach. This ridge is especially strongly ciliated, and appears to assist in the conducting of the food string from the oesophagus around the posterior portion of the stomach to the vicinity of the style head. Along the left of the ridge runs a deeply incised groove, from which opens about half way back, just below the edge of the gastric shield, the single digestive diverticulum (DV.). This leads by a small, round aperture to the large, spirally-coiled posterior lobe of the digestive gland. The smaller anterior lobe, which opens in Serpulorbis just below the mouth of the style caecum, is entirely unrepresented in Stephopoma. The ciliary sorting area (C.S.) is located on the left side of the stomach, and is much simpler in form than in Serpulorbis, consisting of no more than five well-defined ridges, formed by differences in cell height, and running obliquely forward from right to left towards the intestinal opening. The principal movements of food within the living stomach are brought about by the stirring action of the crystalline style, assisted by fine ciliary currents across the tops of the sorting ridges, carrying finer particles transversely over the sorting area. At the same time, coarser particles are carried forward to the intestine by ciliary currents along the intervening grooves. Ciliary currents beat outwards towards the stomach from the longitudinal groove, but at the opening of the diverticulum there is evidently an inward current by which finely divided particles enter the tubules of the digestive gland.
The crystalline style caecum (CM.) has the usual relations. Its epithelium is thrown into several broad transverse folds, densely lined with robust cilia, 12μ in height, with a lateral beat serving for the rotation of the style. The caecum is bounded on the left by two typhlosoles (Figs. 8, 11, DT., VT.) of equal size, projecting bluntly into the stomach behind, on the ventral aspect, and tapering forward to terminate at the dorsal side of the style sac apex. Between the typhlosoles runs the first part of the intestine, a mere narrow cleft, bounded by short-ciliated tracts, beating forwards from the stomach. The two typhosoles are in close contact with the style during life, and along the ventral one runs a tract of darker staining cells representing the style secretion zone. The style (ST.) is a minute rod, 1·0 mm. in length, uniformly narrow and translucent.
Some aspects of digestion in the smaller style-bearing prosobranchs present a problem on which further work is intended. The food in Stephopoma consists mainly of diatoms which are carried intact to the stomach without preliminary digestion of protoplasmic contents, as was seen by opening the stomachs of Stephopoma within a minute or two of collecting. The most frequent diatoms in Milford material were large Coscinodiscus (55μ) and smaller numbers of Pinnularia. Large numbers of empty frustules pass into the intestine, after digestion of their contents, and constitute together with egested particles from the digestive gland, practically the whole of the faeces. The question arises, where and by what means are the diatom contents extracted. In Stephopoma the frustules are obviously too large for ingestion by the epithelium of the digestive gland. They are never
encountered in the digestive tubules, and indeed the large Coscinodiscus are probably too bulky to pass easily through the opening of the diverticulum. Yonge (1926) has shown that in Ostraea, wandering phagocytic cells are responsible for the ingestion of diatoms within the stomach. A preliminary digestion of the diatom contents probably takes place—an example of non-localised intracellular digestion. We may then suppose that after relinquishing the empty frustule, the phagocyte finds its way together with its part-digested contents, to the absorptive epithelia of the digestive diverticulum. In the case of Stephopoma, a careful search was made for phagocytes with ingested diatoms in the stomach contents of a dozen feeding individuals immediately after collecting, but without success, though small phagocytic cells are present as in Struthiolaria (Morton, 1951) in the stomach wall, especially in the subepithelial connective tissue of the sorting area. Probably the large size of the diatoms relative to cell size precludes their ingestion by phagocytes in Stephopoma. This must certainly be the case in the tiny gastropod Rissellopsis varia, investigated by the writer. The stomach is filled by a cord of mucus containing a collection of ten or a dozen diatoms, each about as wide or wider than the digestive diverticulum.
It appears likely that, in these two molluscs at least, extracellular enzyme digestion of diatom contents must take place within the stomach. Yonge (1926) claims that a crystalline style and a free stomach protease cannot normally co-exist. Certainly in Stephopoma the digestive diverticula have all the appearance of an ingesting region, without histological trace of secretion. The claims of Mansour (1946) that the digestive gland of lamellibranchs functions as a holocrine secreting gland, and that the greenish particles entering the stomach are enzymatic, is not upheld by evidence from the similar gland of style-bearing gastropoda. In Struthiolaria, particles carried from the digestive gland to the stomach are incorporated unchanged in the faeces, while in Serpulorbis zelandicus particles of the same type, together with enterochlorophyll spherules, were watched travelling directly by ciliary currents from the diverticulum to the intestine.
The identification of enzyme in such minute amounts of stomach fluid is not easy, and preliminary tests with stained fibrin were inconclusive. Yonge (1926) claims that the crystalline style and a free stomach protease cannot normally co-exist. The style head is gradually broken down in the living stomach, which might conceivably happen by slow digestion as well as by mechanical friction. May there not be in the small diatom feeding gastropods, a weak secretion of free enzymes, including protease, possibly in equilibrium with the rate of style secretion, so that the style is never dissolved in the living animal? The source of such enzyme, if present, requires investigation. The digestive diverticula, despite Mansour's claim, are in the main undoubtedly ingesting organs, returning to the stomach waste products of digestion and excretion.
The diverticulum has no separate ciliated region, passing—on opening from the stomach—directly into glandular cells (Fig. 13) which are of the usual two kinds, digestive and excretory. The ultimate lobules (Fig. 12) are 150μ–175μ in diameter, embedded in vascular connective tissue, and their lumina are round or triangular. The
digestive cells are tall and columnar (45μ) with basal nuclei; the distal halves are clear and apparently vacuolated, with the cytoplasm condensed and granular along the free surface. Proximally each cell contains up to a dozen rounded spherules, greenish yellow in the living animal, and staining more darkly towards the base of the cell. These are freely extruded from the cells and are carried into the stomach. They evidently constitute the residuum after intracellular digestion. The clusters of excretory cells are broad-based and triangular, opening over a restricted distal surface into the lumen. Two types of inclusion are present. One or more cells in each excretory group are entirely filled by a dark brown or jet black mass of enterochlorophyll, elongate-ovoid or somewhat irregular in shape, and discharged at intervals with the smaller greenish digestive cell spherules into the stomach. The large black inclusions are evidently formed by the coalescence of numerous small excretory spherules, resembling the greenish spherules, but black in colour. These are crowded in large numbers in the cytoplasm of excretory cells at the sides of each cluster. Small black particles of the same type occur in the digestive gland of Turritella (personal obs.) which bears a very close resemblance to that of Stephopoma. They are not separately distinguishable in Serpulorbis. The darkly staining particles of the excretory cells are generally held to represent the product of extraction of absorbed chlorophyllous pigments from the blood. Excretory cells are invariably separately developed in the digestive gland of phytophagous gastropods; they do not occur in carnivorous forms, and are much less distinct in the digestive glands of lamelli-branchs.
The intestine (Fig. 7) in Stephopoma is of the simplest structure. The groove between the typhlosoles leads forward to a narrow tube, 150μ in diameter, which loops round the margin of the renal organ, and runs forward as the somewhat wider rectum, with a diameter of 250μ. Long ovoid faecal pellets are formed in the first portion of the tube, by ciliary and muscular action, and are carried forward to the rectum, where they are surrounded by clear mucus secretion, and from time to time discharged one by one from the anus. There is a strong ciliary beat within the rectum towards the anus, and the pellet when evacuated is quickly carried out of the mantle cavity by the exhalant ctenidial current, assisted by the apical tufts of the filaments.
II. Pyxipoma Weldii.
The genus Siliquaria (s. lat.) includes all those vermetids with the shell coiled in a cork-screw shaped spiral, and fissured by a longitudinal slit. There are two New Zealand species, of which the first is generally placed in the separate genus Pyxipoma. Powell (1940) predicted that the neozelanic Pyxipoma would be found to be distinct from the Australian weldii (type locality Tasmania) to which it has hitherto been referred. Though from our knowledge of other New Zealand vermetids, this might seem likely, New Zealand specimens in fact proved identical with Australian material from two separate localities—Seaspray, near Sale, Victoria, and Rhyll, Victoria. Adult shell features vary a good deal in compactness of spire, but the sculpture, and arrangement of growth lines is quite similar. The nucleus is golden brown and its
sculpture of finely granulated spiral striae, is identical in all specimens examined, well-marked in the embryo, though sometimes eroded at the apices of mature shells. The operculum is highly distinctive: it is elevated or dome-shaped, composed of a spirally rolled chitinous band, of some 8 to 10 coils, edged with a single row of short, stiff setae, and enclosing a core of triangular cells, spirally arranged around a central axis, six cells to a whorl. The radula of a neozelanic specimen is illustrated in Figure 22. The central tooth has a single equilateral-triangular cusp, without any other denticles; the laterals have a broad cusp towards the mesial edge, and a single smaller denticle laterally. The marginals are narrow, curved, and without separate serrations.
The second New Zealand siliquariid, S. maoria (Powell, 1940) belongs to the same group as the more heavily built Australian ponderosa; its type locality is off Three Kings, and a second record may now be noted, from a specimen in the Dominion Museum, from Happy Valley, Wellington.
Pyxipoma weldii was obtained alive after an easterly gale at Milford, cast ashore from the sublittoral zone attached to the holdfasts of Ecklonio, where it is typically found embedded in a massive yellowish white sponge. It is invariably an offshore species, frequently turning up in the trawl from a depth of several fathoms, and never found in the littoral zone. A single sponge may be thickly studded with several hundred shells, with the dome-shaped opercula loaded with debris projecting above the surface. The animals dart back quickly within the shell tube, which is completely sealed by the operculum. Unfortunately there was no opportunity to examine living material for ciliary currents; animals were placed in fixative immediately upon collecting.
The structural features of the pallial cavity and alimentary canal (Fig. 2) are noteworthy in placing the animal very close to Stephopoma, in contrast with the vermetids hitherto described. Similar resemblances in mode of life may be inferred from the pallial organs. Pyxipoma is without doubt a ciliary feeder, with long, cirriform gill filaments, shortly attached to the mantle wall, and agreeing in histology with Stephopoma. The adaptation of the gill to form a sweeping fringe as in Stephopoma could not be observed in Pyxipoma, though from the similarity of the filaments may be expected to be present. The pallial cavity is longer than in Stephopoma, and the gill extends through a complete spiral turn occupying the extended last whorl of the shell. The mantle margin is simple, not papillose as in Stephopoma, but finely plicate around its inner edge. Corresponding to the shell fissure, the mantle is slit along the whole length of the right side, between the rectum and the pallial genital duct. It is not easy in Pyxipoma weldii, in which the shell is wholly embedded in sponge, to give any satisfactory explanation of the functions of the shell fissure and pallial slit. In siliquariids in general the slit may reasonably be supposed to have arisen as an adaptation for the rapid expulsion of waste in a ciliary feeder dependent upon a pallial water current. Detritus is probably compacted with mucus from the hypo-branchial gland which borders the slit, and cast off from the gill filaments directly through the slit. The same adaptation may also serve
as a means for the rapid expulsion of water when the head and trunk are sharply retracted into the pallial cavity. In contrast with Stephopoma the operculum is close-fitting, so as to prevent outward flow of water from the aperture of the closed shell. It should be noted that the pallial slit in the siliquariids in no way corresponds to the pallial fissure in the female Serpulorbis, Bivonia and other genera. In these vermetids the shell is never slit, and the pallial fissure is median, to the left of the rectum, never on the right side as in both sexes of siliquariids. In the latter the slit appears to serve the same function as that of the more primitive mesogastropoda; it is not, however, to be looked upon as a survival of a primitive character, but rather as an interesting example of independent acquisition in a highly specialised group.
The food groove in Pyxipoma weldii forms a deep incision (Fig. 2, f.g.) much narrower than in Stephopoma, and bounded along either side by a tall, straight ridge. It can thus be more effectively closed off from the pallial cavity than in Stephopoma, which may help to separate the food groove contents from the detrital particles passed outwards through the pallial slit. The groove extends well forward in front of the right tentacle, and its outer margin curves round sharply to terminate immediately in front of the mouth.
Text Fig. 3—Pyxipoma weldii.
Diagram to show relations of
head, food groove, and glandular
portion of the foot. AF, anterior
marginal flap of foot; BP, opening
of incubatory pouch; F [ unclear: ] ,
terminal portion of food groove;
GF, reduced glandular sole of
foot; MO, mouth; PF, pigmented
lateral region of foot; RO,
Text Fig. 3 illustrates the structure of the head and foot. The proboscis is cream-white in colour, finely rugose and deeply bifid, with a long, vertical mouth slit. The two whitish cephalic tentacles, with eyes at the bases, are longer and better developed than those of Stephopoma. The sole of the foot (gf.) consists of an oval, white, rather transversely furrowed, glandular area, covered with ciliated epithelium. This corresponds to the glandular pad in Stephopoma, and is produced anteriorly, below the mouth, into a broad, rectangular flap (af.) formed by the anterior margin of the sole, and representing
the vertical pre-oral appendage of Stephopoma. Pelseneer (1906) in his diagnosis of Siliquaria, states that the “pedal tentacles are rudimentary.” In Pyxipoma weldii, pedal tentacles do not exist; these structures in vermetids are associated with the presence of the specialised pedal gland, of which there is here no trace. The margins of the anterior flap are sometimes produced into short, blunt lateral processes.
The alimentary canal in Pyxipoma (see Fig. 2) is so like that of Stephopoma as not to need detailed description. The pharynx has the jaws reduced, and the principal means of grasping the food bolus is the small, sharply toothed radula. The oesophagus is a simple, uniformly narrow tube, producing abundant mucus, and lined with cilia which carry the food string to the stomach. There is no longer any trace of dorso-ventral division, or of the occurrence of torsion. The stomach is crescentic or triangular in shape, 0·5 mm. in length, and the crystalline style sac short and stout, with the same relations as in Stephopoma. The digestive gland consists of a single lobe, the diverticulum opening into the stomach as in Stephopoma. The first part of the intestine is cut off from the style sac by paired typhlosoles, and opens forward into the middle intestine, which loops back round the renal organ. This part of the intestine is somewhat sinuous in outline (Fig. 2, int.) as distinct from the simple loop described in Stephopoma. The rectum opens by the anal papilla, which projects slightly across the edge of the pallial slit.
III. Reproductive System.
In both Stephopoma and Pyxipoma the eggs are incubated by the female: the free-swimming veliger stage is eliminated, and the embryos emerge to wander about for a short period before becoming sessile. Unlike Serpulorbis and previously described vermetids, the eggs are not enclosed in common capsules attached to the inside of the shell. Each egg is contained in a separate capsule, and in Stephopoma roseum the ova—some 10–15 in number—are retained freely within the mantle cavity of the female, crawling out from the pallial aperture after hatching from the egg membrane. In the male Stephopoma, the testis is composed of two or three saccular lobules, drained by a narrow gonadial duct, convoluted, and lined with low-celled, non-glandular epithelium. The sperms in the gonadial duct are aggregated in dark-staining bundles 7μ across. There is no prostate, and the sperms leave the male aperture far back in the pallial cavity, being carried forward by cilia along the right margin of the food groove and discharged with the exhalant current. As in other current fertilised prosobranchs, cf. Turritella (Fretter, 1946); Serpulorbis (Morton, 1949b), the pallial genital duct is widely open ventrally for the reception of water-borne sperms. The small ovarian duct (Fig. 7, ov.) capable of great distention during passage of eggs, opens into a short, albumen gland (al.) dorso-ventrally compressed and situated between the oesophagus and the intestine. The dorsal and ventral walls are strongly ciliated, and possess columnar gland cells (75μ tall) filled with protein spherules secreted round the egg, and staining orange yellow in Van Giesen's. The anterior half of the albumen gland remains open to the pallial cavity by a narrow slit along the left side
of the lumen. Towards the capsule gland its cell contents begin to change to a mucoid secretion, staining deeply purple with haematoxylin. The capsule gland itself (cp.) is a long, straight tube, with dorsoventral lumen open to the pallial cavity, with white, glandular walls, deeply furrowed by vertical rugae. The gland cells are uniformly filled with mucoid secretion, lightly staining with haematoxylin. The eggs are surrounded by a tough mucoid membrane of the same staining reaction as the capsule gland cells. Fig. 20 is of an embryo about to break through the capsule membrane. Towards the hatching stage, the embryo is equipped with a large circular operculum, with the margin unornamented. By the sharp edge of the operculum the embryo breaks through the capsule membrane. At the crawling stage depicted in Fig. 21, the embryo shell is complete, with its wide trumpet-like mouth. The foot is long and narrow, rounded behind, with finely ciliated, glandular sole, and somewhat more expanded, strongly ciliated in front. The anterior margin of the foot is incised by a groove, into which open a cluster of mucus-secreting gland cells (Fig. 21, pg.). The cephalic tentacles are long and diverging with eyes well developed at the bases. The gill filaments are narrow and club-shaped, relatively few in number, and the food groove and exhalant siphonal rudiments are already present. The shell does not become attached till a slightly later stage, and as in Serpulorbis, the crawling embryo adds a portion of the first unsculptured whorl to the completed nuclear shell.
Pyxipoma differs little from Stephopoma in the arrangement of the genital ducts. The capsule gland opens at the bottom of a deep groove, overhung by a broad fold, by which the genital duct is shut off from the strong current passing through the pallial slit. The chief feature of interest in Pyxipoma is the possession of a spacious incubatory pouch, lying within the trunk cavity. This opens forward by a wide duct with its aperture immediately beneath the right margin of the foot (Text Fig. 3, bp.) just in front of the termination of the food groove. The eggs pass forward by a ciliated furrow along the right side of the food groove, and enter the incubatory pouch, from which they emerge as creeping embryos to wander about before attachment. The brood is larger than in Stephopoma—about 100–150—and the embryos smaller, 450μ in shell diameter. The course of development is otherwise similar. The embryonic operculum is simple, flat and translucent. The development of a brood pouch with a cephalic opening, and the retention of the primitive ciliated oviducal groove, has occurred also in the freshwater mesogastropods Tanganyicia and Melania (Moore, 1899). There seems to be no point in Moore's contention that this is a highly primitive character. Ovoviviparity is always an advanced feature in mesogastropods, and incubatory devices of various types have developed separately in a number of highly specialised forms.
From the present results, together with the preceding account of Serpulorbis zelandicus, it seems clear that the family Vermetidae as classically recognised is naturally divisible into at least two sections without very close relationships with each other. The two groups are broadly similar in having adopted a sessile mode of life with an
unwound or loosely coiled shell, and in development of ciliary feeding, accompanied by a crystalline style in the gut. In structural details, however, there is no very close correspondence, either in the foot and pallial region, or in the alimentary canal. The first group includes “Vermetus” (s.str.), Serpulorbis, Aletes, Spiroglyphus, and Bivonia, and probably also the New Zealand genus Novastoa, of which a detailed account is in the course of preparation. In the second group may be associated together the smaller operculate vermetids of the genera Stephopoma, Siliquaria and Pyxipoma. The following summary of structural differences marks a fundamental line of division in the Vermetidae.
|Vermetus-Serpulorbis Group.||Stephopoma-Siliquaria Group.|
|Shell irregularly coiled, fused substratum, septate. Animal vermiform.||Shell loosely spiral, embedded in substratum, non-septate. Animal never vermiform.|
|Operoulum simple, becoming reduced and lost.||Operculum always well developed, variously setose.|
|Feeding by ciliary means and by mucus traps.||Ciliary feeding. Mucus traps never formed.|
|Gill filaments primitively triangular.||Gill filaments specialised, rod-like or cirriform.|
|Gill never protruded in feeding.||Gill may be protruded to serve as sweeping net.|
|Pedal Gland highly developed.||Pedal Gland absent.|
|Sole of foot greatly reduced.||Sole of foot less reduced, anterior margin variously specialised.|
|Pedal tentacles always present.||Pedal tentacles never present.|
|Endostyle not developed.||Endostyle well developed.|
|Food tract a wide glandular strip.||Food groove a narrow gutter.|
|Mantle medianly slit in female.||Mantle never with median slit, but slit along right side in both sexes in siliquariids.|
|Eggs in groups in capsules attached to interior of shell.||Eggs singly in capsules, incubated in brood pouch or retained freely in pallial cavity.|
|Jaws well developed.||Jaws reduced.|
|Oesophagus anteriorly wide and dilated.||Oesophagus a narrow tube throughout.|
|Anterior lobe of digestive gland retained.||Anterior lobe of digestive gland lost.|
|Proximal part of intestine spacious.||Proximal part of intestine a narrow groove.|
|Stomach differs in structural details in the two groups.|
The origin in the two groups of the sessile habit and the uncoiled shell is possibly to be sought independently among separate stocks of free-moving style bearing mesogastropods. On the one hand the main evolutionary trend has been from ciliary feeding towards mucous trap feeding, with the accompanying enlargement of the pedal gland and tentacles and reduction of the gill. In the second group, as typified by Stephopoma, ciliary feeding adaptations are more perfect, and evolution seems to lead finally to the sweeping net mode of food capture. Neither pedal gland nor pedal tentacles are present.
The comparative table also brings out the close resemblance that exists between Stephopoma and the siliquariids (as typified by Pyxipoma). Thiele (1931) conservatively divided the Vermetidae into two large “generic” sections—Vermetus, comprising nine sub-generic groups, including among them Stephopoma, each better recognised as genera—and Tenagodus (equals Siliquaria s. lat.) distinguished by the longitudinally fissured shell. This shell distinction is now seen
to be of less fundamental importance than has been hitherto supposed. Important similarities have been pointed out between Stephopoma and the siliquariids in the head, foot, pallial cavity, and the digestive and reproductive systems, indicating that these vermetids should be placed together in a single natural group. The principal diagnostic feature is the embryo shell, which is exactly alike in shape in Stephopoma and Pyxipoma, consisting of 1½ whorls coiled in an almost plane spiral, of nautiloid or limacinid shape. The mouth is wide, circular and trumpet shaped, with the peristome projecting freely around the inception of the adult shell. The embryonic operculum is in both cases a simple circular disc. The adult shell is coiled in a regularly increasing corkscrew spiral, generally embedded in or loosely attached to the substratum, never firmly cemented as in Serpulorbis, Aletes or Spiroglyphus. The radula (Figs. 22, 23) is at once distinguishable from that of previously described vermetids. The operculum is formed of a spirally coiled, setose band, a flat spiral with complex setae in Stephopoma, and in Pyxipoma elevated to form a tall dome, with the margin simply setose. The full significance of the shell fissure in the siliquariids remains to be worked out with observations on living material: the slit would seem to have arisen as a more efficient adaptation for the removal of rejected waste material from the mantle cavity.
As regards the Stephopoma-Siliquaria section of the vermetids, the closest relationships of these genera would appear to be with the Turritellidae. While allowing for the convergent adaptations that are especially likely to exist in specialised groups of ciliary feeders, we may recognize in the account by Graham (1939) of Turritella communis several important resemblances to Stephopoma roseum as described above. In Turritella and Stephopoma, the pallial organs are similar, in the endostyle, hypobranchial gland and food groove. In both cases the gill filaments are narrow and linear, in adaptation to ciliary feeding; in Stephopoma, however, they are especially slender and cirriform, in relation to the sweeping fringe mode of feeding. The mantle margin in Stephopoma is fringed with simple papilliform tentacles which are present also in Turritella, which has as well an additional series of pinnate guarding tentacles, protecting the entrance to the pallial cavity. These are unrepresented in Stephopoma, being perhaps replaced functionally to some extent by the long opercular setae. Turritella retains a much more generalised structure of the foot being free-moving and using the sole for creeping in the normal fashion; it is significant that the operculum is fringed with simple setae, perhaps foreshadowing the condition found in Stephopoma and Siliquaria. The exhalent siphonal appendages described by Graham in Turritella are represented by the triangular siphonal tubercle on the right side of the foot in Stephopoma. Further, in both groups the snout is short, the jaws reduced, and the food bolus is seized by the radula, while the structure of the pharynx, salivary glands and oesophagus is almost identical. The stomach of Turritella and Stephopoma are extremely similar, in the shape and proportions of the style caeum, the relations of the sorting area, and the presence of a crescentic fold, with a deep groove from which opens the single digestive diverticulum. In both forms the intestine originates as a narrow groove between paired typhlosoles, and the digestive gland shows detailed
similarity in histological structure.
Such details of the animal of Vermicularia as were provided by Mörch (1861) also suggest an affinity with Stephopoma: pedal tentacles are not mentioned, and the mantle is stated to be fringed at its margin with short filaments. The cephalic tentacles are short and conical. The condition of the gill is not described, but the presence of a well-developed food groove is indicated by reference to an elevated ridge that “runs along the back, becomes flattened into a membrane at the head, and passes round under the right tentacle forming a kind of canal, near which is the anus”. A relationship between the Turritellidae and the corkscrew-shaped vermetids is rather strikingly suggested by the developmental history of Vermicularia, as for example in V. spirata illustrated in Fig. 24, which begins life as a close-coiled turreted spiral almost indistinguishable from a Turritella. An examination of living material of species of Vermicularia, and in particular a comparison of animal structure, apex of shell and radula with the Turritellidae is much to be desired.
If, as has been suggested, it is considered more convenient to regard the genera Stephopoma, Pyxipoma and Siliquaria as a separate family from the Vermetidae, s. str., this group might be best placed alongside the Turritellidae, and would be known, from its oldest genus, as the Siliquariidae.
Synonymic List of the Recent New Zealand Vermetidae and Siliquariidae
Family Vermetidae d'Orbigny
Genus Serpulorbis Sasso 1827
1. S. zelandicus (Quoy and Gaimard) 1834
|1834||Vermetus zelandicus Q. and G. Voy. Astrol., iii, 293, pl. 67, f. 16–17.|
|1858||Cladopoda zelandica (Q. and G.) Mörch, J. de Conch., vii, 349.|
|1859||Vermetus novac-zelandiae (Q. and G.) Gray, Figs., Moll. Anim., ii, 28, pl. 56, f. 6.|
|1886||Vermetus (Thylacodes) zelandicus Gray. Tryon and Pilsbry, Man. Conch (1), viii, 182, pl. 54, f. 81.|
|1904||Vermicularia zelandica (Q. and G.) Hutton, Index Faunae N.Z., 76.|
|1913||Serpulorbis sipho (Lamk.) Suter, Man. N.Z. Moll., 259, pl. 40, f. 9 (in part).|
|1927||Vermicularia sipho (Lamk.) Finlay, Trans. N.Z. Inst., 57.|
|1946||Vermicularia sipho (Lamk.) Powell, Check List Shellfish of N.Z.|
|1951||Serpulorbis zelandicus (Q. and G.) Morton, Trans. Roy. Soc. N.Z., vol. 79.|
2. S. aotearoicus Morton
1951 Serpulorbis aotearoious Morton, Trans. Roy. Soc. N.Z., vol. 79, 5.
Genus Novastoa Finlay 1927
3. N. lamellosa (Hutton) 1873
|1873||Siphonium lamellosum Hutton, Cat. Mar. Moll., 30.|
|1886||Vermetus (Siphonium) lamellosus Hutton. Tryon and Pilsbry, Man. Conch. (1), viii, 184.|
|1904||Vermicularia lamellosa Hutton, Index Faunae N.Z., 76.|
|1913||Siphonium lamellosum Hutton, 1873. Suter, Man. N.Z. Moll., 261, pl. 40, f. 11.|
|1928||Novastoa zelandica (Q. and G.) Finlay, Trans. N.Z. Inst., 59.|
|1946||Novastoa zelandica (Q. and G.) Powell, Check List Shellfish of N.Z., 2nd edition.|
|1951||Novastoa lamellosa (Hutton) Morton, Trans. Roy. Soc. N.Z., vol. 79, 43.|
Genus Siliquaria Bruguiere 1789
4. S. maoria Powell
1940 Siliquaria maoria Powell, Trans. Roy. Soc. N.Z., 70, 231.
Genus Pyxipoma Mörch 1860
5. P. weldii (Tenison-Woods)
|1875||Siliquaria weldii Tenison-Woods, P.R.S. Tasm., 44.|
|1886||Siliquaria weldii Tenison-Woods, Tryon and Pilsbry, Man. Conch. (1), ix, 191, pl. 58, f. 28.|
|1904||Tenagodes weldii (Tenison-Woods) Hutton, Index Faunae N.Z., 76.|
|1913||Siliquaria weldii Tenison-Woods, Suter, Man. N.Z. Moll., 264, pl. 39, f. 15.|
|1946||Pyxipoma weldii (Tenison-Woods) Powell, Check List Shellfish of N.Z., 72, pl. 14, f. 26.|
Genus Stephopoma Mörch 1860
6. S. roseum (Q. and G.) 1834
|1834||Vermetus roseus Q. and G., Voy. Astrol., iii, 300, pl. 67, f. 20–23.|
|1861||Stephopoma roseum (Q. and G.) Mörch, Proc. Zool. Soc. Lond., 150.|
|1873||Siliquaria laevigata (Lamk.) Hutton, Cat. Mar. Moll., 31, not of Lamarck.|
|1880||Stephopoma roseum (Q. and G.) Hutton, Man. N.Z. Moll., 85.|
|1904||Vermicularia rosea (Q. and G.) Hutton, Index Faunae N.Z., 76.|
|1905||Vermicularia (Stephopoma) nucleogranosa (Verco) Suter, Trans. N.Z. Inst., 38, 328, not of Verco.|
|1913||Stephopoma nucleogranosum Verco. Suter, Man. N.Z. Moll., 262.|
|1927||Lilax nucleogranosum (Verco) Finlay, Trans. N.Z. Inst., 57.|
|1951||Stephopoma roseum (Q. and G.) Morton, Trans. Roy. Soc. N.Z., 79, 20.|
7. “Vermicularia” maoriana Powell
1937 Vermicularia maoriana Powell, Discovery Repts., 15, 153–222.
8. “Magilus” inident.
1928 Finlay, Rec. Moll. Chatham Is., Trans. N.Z. Inst., 59, 232–286.
The writer is deeply indebted to Mr. A. W. B. Powell for generous advice and encouragement, as regards both the Vermetidae and the mollusca at large, and for the frequent loan of specimens; to the Auckland Museum for making available specimens from the Finlay Collection; to the New Zealand Geological Survey for the loan of material from the Suter Collection; and to Mr. R. K. Dell, Conchologist, Dominion Museum, Miss J. Hope McPherson, of the National Museum of Victoria, and Mr. B. C. Cotton, of the South Australian Museum, for generous help in securing vermetid shells and animals. Mr. D. Whillans was kind enough to prepare the photomicrographs in Plate 2 of this part.
The sole neozelanic species of the genus Stephopoma is shown to be roseum (Q. and G.). The South Australian nucleogranosum is not represented, and shows slight but valid differences from roseum. Radular, nuclear and opercular characters are considered, and it is suggested that Finlay's genus Lilax is not required. The mode of life,
and feeding and cleansing mechanisms of Stephopoma roseum are described in detail, with the anatomy of the head, foot and pallial cavity. The species is a ciliary feeder, having but few detailed resemblances to the vermetids previously described in life. There is a ciliary method of food collection by the gill filaments most resembling that of Crepidula, and in addition, a supplementary mode of feeding by pro-extruding the anterior fourth of the gill from the pallial cavity to form a sweeping net of cirrus-like filaments. This method of feeding has not been previously described in gastropods. A pedal mucus gland for the extrusion of food-collecting mucous traps is not developed in Stephopoma. The alimentary canal differs to a considerable extent from that of previously described vermetids. An account is given of its structure, histology, and function, with suggestions as to the nature of the digestive mechanism.
Pyxipoma weldii, a member of the siliquariid group of Vermetidae, is validly represented in New Zealand. The operculum, radula, and embryonic shell are described and figured. The anatomy of the head, foot and pallial cavity is closely comparable with that of Stephopoma, and the significance of the pallial slit and shell fissure is considered. The female reproductive system in Stephopoma and Pyxipoma is described, with details of histology. In Pyxipoma there is a spacious brood pouch in the trunk of the female, opening anteriorly at the base of the right tentacle.
The resemblances between Stephopoma and Pyxipoma are emphasised and a tabular summary is provided, showing the dissimilarity between these genera and previously described Vermetidae. The family Vermetidae as at present constituted is concluded to be diphylectic and a separate family the Siliquariidae is proposed. The diagnostic characters of this family are listed, and its relationships with the Turritellidae are discussed. The genus Vermicularia is suggested to be most closely allied to the Siliquariidae.
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