Umbonium vestiarium, a filter-feeding trochid



U. vestiarium, a dominant species on the seaward side of sand flats at the mouth of the River Vellar, S. India, collects food by filtering the palliai stream of water as it glides over the surface of the sand or is buried except for its tentacles, eye stalks and siphons. It may remain stationary or creep when buried, but feeds only when conditions are quiet. Inhalant and exhalant siphons are formed by folding of the neck lobes of the epipodium; elsewhere the entrance to the mantle cavity is blocked by the head. The inhalant current is filtered by siphonal tentacles and then passes over the osphradium which encircles the base of the siphon. The gill is monopectinate with the axis, which is fused to the mantle throughout its length, shifted to the right so that the prebranchial chamber of the mantle cavity is large; it can be reduced by muscles in the mantle skirt and the gill displaced from the feeding position. The gill filaments are filiform and elongated and their tips overarch a food groove, except anteriorly. Secretion for trapping food is produced by an endostyle and hypobranchial glands. The food cord, formed by the combined action of cilia on the tips of the filaments and dorsal body wall, is directed to the lateral margin of the right eye stalk. It passes beneath the stalk towards the mouth which is at the end of a very small tubular snout hidden between the bases of the siphons. When food in suspension is scarce, organic matter is collected from the substratum, its supply to the vicinity of the mouth being regulated by the activity of the right tentacle and the foot. Odontophore, salivary and oesophageal glands are reduced. There is no crystalline style; the stomach has large sorting areas and a small gastric shield. A valve prevents entry of paniculate matter to the single duct of the digestive gland. Modifications of shell and foot facilitate burrowing in the uncompacted sand and enable the snail to resist displacement when conditions are adverse. The animal moves easily through soft substrata, but with difficulty over a hard one.