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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laterals form a set of tiny grappling hooks by which a bolus is detached from the food groove mucus cord, and withdrawn into the mouth.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
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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
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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
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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.