Cephalaspidea is an Order of marine gastropods molluscs with approximately 650 living species worldwide. The systematic composition of the group comprises 12 families and about 40–50 genera worldwide. The first studies about these molluscs in Norway date back to the first half of the 19th century and presently (2014) it is estimated that between 40 to 45 species occur in the country.
The typical spiralled gastropod shell is absent in the cephalaspids; most species have a bubble-shaped shell (because only one whorl is completely visible) with a short or involute spire. Some groups even lack a shell (e.g. Philinoglossidae) or have a reduced internal shell (e.g. Aglaja, Chelidonura, Navanax). The shell can be strongly calcified (e.g. Scaphander) but often it is thin, fragile, and translucent (e.g. Philine).
Most species are smaller than 1 cm, yet several genera have in average larger species (e.g. Bulla, Scaphander) and in Norway it is possible to find the largest species in the World – Scaphander lignarius – with a shell that can reach up to 7 cm.
The body of these animals varies from whitish to brownish but some families, particularly in tropical regions, have members with vibrant colours (e.g. Aglajidae, Gastropteridae, Haminoeidae).
Cephalaspidean snails are hermaphroditic with both male and female genital systems but they practice cross-fertilization with mating involving the participation of two animals. These snails live from shallow costal areas down to the deep-sea and inhabit soft bottoms of mud and sand, seagrass meadows, and hard substrates with algae. Most cephalaspids use their shovel shaped head to burrow in the sediment where they search for food and all have a strong muscular gizzard equipped with three hard plates to crash and grind their prey. The diet is formed mostly by diatoms, filamentous algae, foraminiferans, polychaete worms, and smaller molluscs.
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