Head-Foot Region

The foot is short and the sole of the foot is white to yellowish-dirty white in colour (Houbrick 1987). At the dorsal-posterior end of the foot is a large, horny, lenticular and paucispiral operculum with subterminal nucleus (Houbrick 1987).

Foot of Planaxis sulcatus viewed ventrally.
Note the yellowish-dirty white colouration
of the sole.

The downward pointing snout is flared into a crescent-shaped oral hood at its tip; a feature that is distinctive to all planaxid taxa (Houbrick 1987). The mouth is a longitudinal slit in the concave ventral portion of the oral hood (Houbrick 1987).

The mantle edge is smooth and slightly scalloped (Houbrick 1987).

The osphradium is an olfactory organ used to sense and monitor quality of water (i.e. chemicals, sediments) entering the mantle cavity (Ruppert, Fox & Barnes 2004). In P sulcatus, the osphradium is a simple, thin ridge flanked by a pair of thin, densely ciliated strips (Houbrick 1987).

The ctenidium (gills) is broad, composed of shallow, triangular filaments, extends the full length of the mantle cavity (Houbrick 1987). It is used as the respiratory organ in P. sulcatus and its structure is typical of all cerithiaceans (Houbrick 1987; Ruppert, Fox & Barnes 2004). There is a raised ridge at the basal side of the ctenidium adjacent to the osphradium (Houbrick 1987).

The hypobranchial gland is glandular structure responsible for producing mucus and biologically active compounds in many molluscs (Ruppert, Fox & Barnes 2004). The hypobranchial gland of P. sulcatus is wide, thick and whitish in colour (Houbrick 1987). It is divided into raised transverse ridges and lie adjacent to the distal end of the osphradium where it curves into the inhalant siphon (Houbrick 1987). The hypobranchial gland of P. sulcatus is very well developed like all planaxid taxa and capable of producing copious amounts of mucus (Houbrick 1987).

As all gastropods undergoes torsion or twisting 180° counter clockwise of the visceral mass, shell, mantle and mantle cavity during development, the digestive tract is looped into a U-shaped where the stomach is posterior and dorsal whereas the mouth and anus are anterior and ventral (Ruppert, Fox & Barnes 2004). From the above diagram showing the mantle cavity organs of P. sulcatus, the digestive tract can be trace as yellowish white tubes coiling around the visceral mass. The stomach is visible as a dark brown patch in the ventral side, while the rectum is located above the head and can be easily recognisable as being the black tube to the right of the ctenidium on the dorsal side of P. sulcatus. The rectum appears black due to accumulation of transversely stacked fecal pellets often observed in the intestines of P. sulcatus. The distal end of the pallial oviduct opens anterior to the rectum (Houbrick 1987).