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
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
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.