 Evidence for any such direction in evolution is however hard to acquire.For example, in the gastropods, the snail-type shell is always built as a tube that grows both in length and in diameter; selection has created a wide variety of shell shapes such as flat spirals, cowries and tall turret spirals within these constraints.Among the centipedes, the Lithobiomorpha always have 15 trunk segments as adults, probably the result of a developmental bias towards an odd number of trunk segments.