This group is relatively poorly represented in the UK with only two species, one of which only occurs in deep water. Species in this group possess both cardinal and lateral teeth and a display a pallial line which is not indented by a sinus.
They feed by filtering plankton from the water through a pair of siphons (which are joined through-out their length). The sexes are separate with eggs and sperm released into the water in spring where fertilisation occurs. The larvae have a short plantonic phase before settling and developing into adults.