 Each zooid lies in a rigid rectangular box called a zooecium up to long and wide.These are fitted together in a regular pattern, radiating out from the position in which the founding zooid settled to start the colony, each daughter zooid's head being further away from the centre of the colony than its foot.The upper surface of the zooecium has an opening called the aperture at the head end, and through this the zooid's lophophore is extended to feed.