Fig. 9

Sequence diagram to explain the tube formation and moulting in Selkirkia (Cambrian). a Worm attached to its body (just before moulting). b Ecdysis: freshly moulted worm splits from its old cuticle (black arrow) (tube + thinner cuticle all over the body). At that stage, there is a gap between the worm and its tube. The worm can move within it. c Worm grows in size within its tube, mainly in length, allowing the body including trunk to protrude outside. d Worms leaves and discards its old protective tube. At that stage, it is well protected by a new cuticle but the tube has not formed yet. e Worm secretes a new tube around its trunk as in a. Abbreviation: oc, old cuticle. Note that Selkirkia is seen from above, and although Selkirkia was mainly epibenthic, moulting may have occurred slightly below the water/sediment interface