Echinoderms are invertebrate animals that include sea stars ("starfishes), echinoids (sea urchins), sea cucumbers and crinoids (sea lilies). They live only in seas and oceans. The scientific name is Echinodermata, which means "spiny-skinned". Echinoderms have plates or spines of calcium carbonate embedded in the skin. All echinoderms have tiny, tubular structures called tube feet that may act as walking legs, or are used in feeding or in extracting oxygen from seawater.
Arthropods are the most important animals alive today. About 75% of living animal species are arthropods, and most arthropods are insects. Other arthropods include the crustaceans (crabs, lobsters, etc.), arachnids (spiders, scorpions , etc.) myriapods (centipedes and millipedes) and the extinct trilobites. All arthropods have a body made up of a series of segments and they have an external shell that acts as protective armor. Arthropods grow by molting. They shed their old skeletons and replace them with new ones as their bodies become larger.
Cnidarians include jellyfishes, corals and anemones. They are carnivores that capture their prey with tentacles that have stinging cells. The life cycles of many cnidarians are complex, with a swimming phase (medusa) and a phase attached to the sea floor (polyp). Jellyfishes spend most of their lives as the swimming phase. Corals and anemones spend their entire lives as polyps. Corals may be solitary (one polyp) or colonial (many polyps living together). Aside from the stony corals, modern cnidarians lack a hard skeleton and are unlikely to be fossilized.
Even if you haven't seen a live mollusc, you've probably eaten one as they are on the menu at most seafood restaurants. Molluscs include gastropods (snails and slugs), bivalves (oysters, clams, scallops, mussels, etc.) and cephalopods (octopuses, squids and the chambered Nautilus). The scientific name is Mollusca, and it means "soft-bodied". However many molluscs have hard shells that become fossilized easily. Most have a muscular foot for slow, creeping movement or burrowing, although cephalopods are swimmers. In other ways, molluscs are variable. Cephalopods have large brains and excellent eyesight, but in bivalves, the head is little more than a mouth.