Segmented worms are soft-bodied animals, and the largest group are the marine polychaete worms that have paddle-shaped projections and bundles of spines projecting from the body segments. Some polychaete worms move about freely on, or in, the seabed, or swim actively in the water. These worms usually have a well-developed head, with eyes, sensory tentacles and strong jaws that they use to capture prey. Other polychaetes are sedentary and live in tubes constructed from calcium carbonate, sand grains or secreted material. Many of these worms have specialised arrays of feeding tentacles that filter passing plankton and organic material from the water, whereas other species feed on organic material in the sediment. Polychaete worms reproduce sexually, producing eggs or sperm that may be shed into the sea for fertilisation, resulting in a planktonic larval stage. Other species brood their young in special chambers.