Digging lugworm question

Just a qestion on digging lugworm , when you see the circular depression after a squiggle does this mean black lug??

Black Lug is identified by a neat cast but you wont see a blow hole beside the cast like the Blow Lug. You might see what appears to be a blow hole in the centre of the cast. Black lug will be found usually only at extreme low tides and dug using a long thin spade or a pump.