Alimentary Canal

The digestive system of Novastoa lamellosa conforms to the general vermetid plan, described by the writer for Serpulorbis. The chief features of comparative importance are the radula (Text Fig. 1, a), and the structure of the stomach and style sac, which form the most complex functional region of the gut.

As with the evidence of the apex and the operculum, the radula affords little doubt as to the close relationship of Novastoa and Spiroglyphus. In Novastoa lamellosa the central tooth is produced posterolaterally into slightly curved, back-directed wings, fitting into concavities in the mesial edges of the laterals. The posterior margin has a median obtuse point, being shallowly emarginate on either side. The anterior edge bears a long median cusp, flanked at each side by a row of three smaller denticles. The lateral tooth has a large cusp the size of the central cusp, with one smaller denticle to the mesial side and a row of three denticles laterally. The inner marginal is equipped with a spur-like forward-projecting cusp a short way behind the tip, with two shorter denticles further back. The outer marginal possesses the sharp, spur-like cusp alone. In Spiroglyphus (Text Fig. 1, b) the postero-lateral wings of the central are stout and peg-like, and the posterior margin is edged with a thin triangular flange. The long median cusp is flanked by four denticles, not three as in Novastoa: a larger denticle flanking the cusp on either side, with an outer row of three short denticles, which may occasionally increase to four. The laterals and marginals are scarcely to be distinguished from those of Novastoa.

The details of the stomach and visceral mass resemble most closely the description by Yonge of “Vermetus” novae-hollandiae. The stomach (Fig. 5) is a large cylindrical chamber, with a shorter style caecum (s.cm.) lying in front. The digestive gland is divided into two parts: a smaller anterior lobe (dg.r.) on the right side, close to the style caecum, and a larger posterior lobe opening from the stomach behind. The posterior lobe (dg.l.) is not greatly elongate and vermiform as in Serpulorbis but shorter, and bluntly tipped, as figured for V. novae-hollandiae, filling the shell tube back to the first septum. The stomach wall is thick, but contains little muscular tissue, being made up of an opaque parenchymatous connective tissue, apparently storing metabolic reserves. The style caecum is separated from the proximal part of the intestine by a single wide, low typhlosole (ty.) running along the ventral side. It opens into the spherical anterior portion of the stomach, the anterior digestive diverticulum being located just beneath the mouth of the caecum. The style rotates against the gastric shield (g.sh.), a transparent recurved plate of cuticle, wide and thin at its margin. The small and rounded gastric shield figured by Yonge in “Vermetus” novae-hollandiae, probably corresponds to the epithelial ridge secreting the actual substance of the shield, or to the strongly projecting fold of epithelium separating the oesophageal opening on the left from the gastric shield. The crystalline style (s.) is retained without resorption through the period between tides when the animals are exposed and feeding is interrupted. It is a narrow, very delicate, flexible rod, small in relation to the size of the animal. Its colour is golden-brown, and diatoms are frequently