The Scaphopods or "Tusk shells" are an unusual group of molluscs that are rarely found alive - except by divers or dredging - but often the empty shells are washed up on the shore. The curved shell (shaped like an elephants tusk - hence the colloquial name) is open an both ends and they live buried in sediment with then thinner end of the shell projecting in to the water. Evidence from fossils shows that the group first evolved in the Devonian period - some 400 million years ago. They burrow into the sediment using a muscualr foot very similar to that found in bivalves. The foot is pushed down into the sediment then the lobes are extended to act as an achor and the shell is pulled down by contraction of the muscles.
At th bottom end of the shell, next to the foot there are bunches of tentacles (known as captaculae) that search for organic material and particularly foraminiferans (small protozoans with a hard external covering). These particles are passed along the tentacle to the mouth where a radula (a ribbon like structure) passes them to the stomach. Here they are crushed and digested.
Tusk shells do not have gills but rather parts of mantle have a good supply of blood and act as respiratory surfaces. Water is slowly drawn into the mantle cavity probably by a combination of cilia but mainly through muscular movements of the foot.
The males and females are separate with eggs and sperm released freely in to the water where fertilisation occurs. The fertilised egg develps into a trochophore larvae and later into a veliger larvae. Interstingly the shell at this stage consists of two parts - possibly indicating some sort of evolutionary link to the bivalves. The veliger larave setlles on the bottom and develops into a mature animal.