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-
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.