A trio of biologists from Stanford University in the US grew acorn worms (Schizocardium californicum) from larvae to adult (pictured above).
They found that in the worms, activity of certain genes that would lead to the development of a trunk are delayed. So, during the larval stage, the worms are basically swimming heads.
As they continue to grow, the acorn worms eventually undergo a metamorphosis to their adult body plan. At this point, the genes that regulate the development of the trunk activate and the worms begin to develop the long body found in adults, which eventually grows to about 40 centimetres over the span of several years.
The work was published in Current Biology.