Sand dollars are species of flat, burrowing sea urchins belonging to the order Clypeasteroida. Some species within the order, not quite as flat, are known as sea biscuits. Sand dollars can also be called "sand cakes" or "cake urchins".
Sand dollar
Examples of Rotulidae
Encope emarginata (aboral and oral faces) by Ernst Haeckel (1904)
Clypeaster rosaceus (aboral and oral faces) by Ernst Haeckel (1904)
Benthos, also known as benthon, is the community of organisms that live on, in, or near the bottom of a sea, river, lake, or stream, also known as the benthic zone. This community lives in or near marine or freshwater sedimentary environments, from tidal pools along the foreshore, out to the continental shelf, and then down to the abyssal depths.
Seaweed and two chitons in a tide pool
Seagrass
Echinoderms
Aquatic worm, snail, midge larvae and amphipods