Brittle stars and certain groups of sea stars lack an anus, however, and vent food via the mouth once their digestion is complete. In most echinoderms there is a relatively straightforward digestive process with food entering the mouth and vacating via the anus. Pedicellariae serve in a variety of functions, ranging from defense (with some species bearing venomous pedicellariae) to assisting in food capture in some species of sea stars, such as in deep-sea brisingid group, which use them to grab crustacean food. In addition to spines, urchins and sea stars both possess pedicellariae (singular pedicellaria) which are jaw-like structures often at the end of stalks. The long, spiked ossicles of Coelopleurus exquisitus, a sea urchin species from New Caledonia in the South Pacific.Įchinoderms possess an unusual suite of appendages on their skeletons, which are used for movement, interaction, and defense. Groups such as sea stars display skeletons arranged in a fashion intermediate between these two extremes. The opposite of this is observed in sea cucumbers, which only possess ossicles as small, separate pieces in a soft worm-like body, viewable only with a microscope. The ossicles can be long and sharp-as you see in sea urchins- or short and hair-like as seen on sand dollars and their relatives). In sea urchins (which includes sand dollars), ossicles are tightly fused into sphere or flattened cake-like shapes with articulated, projecting spines.

Ossicles come in many shapes, which creates the diversity of complicated body types observed in echinoderms. Similar to a complicated 3-D puzzle, these pieces are infused with tissue and covered by an epidermis, or skin like covering.

Sea lilies, however, obtain water through many pores that cover their body.Įchinoderms possess a unique internal skeleton (or endoskeleton) composed of thousands to millions of calcium carbonate components known as ossicles. In most echinoderms, water enters the body through the tube foot grooves and via a plate called the madreporite, which is often visible to the naked eye on the top side of sea stars. This system uses water pressure to help give the animal shape and help them travel. While humans rely on a network of vessels filled with blood, echinoderms possess a complicated series of tubes which uses seawater to convey nutrients and gases throughout their body. The ochre sea star ( Pisaster ochraceus) exhibits the classic five-fold symmetry.Īnother characteristic of all echinoderms is their water vascular system.

Do sponges havelegss to move