In one particular branch of the family tree , the shell of conchiferans is thought to have evolved from the spicules ( small spines ) of aplacophorans ; .
This is difficult to reconcile with the embryological origins of spicules .
In one particular branch of the family tree , the shell of conchiferans is thought to have evolved from the spicules ( small spines ) of aplacophorans ; but this is difficult to reconcile with the embryological origins of spicules .