The term Sand dollar refers to species of extremely flattened, burrowing echinoids belonging to the order

Order (biology)

In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

 Clypeasteroida. Some species within the order, not quite as flat, are known as sea biscuits. Related creatures include the sea urchins, sea cucumber

Sea cucumber

Sea cucumbers are echinoderms from the class Holothuroidea.They are marine animals with a leathery skin and an elongated body containing a single, branched gonad. Sea cucumbers are found on the sea floor worldwide. There are a number of holothurian species and genera, many of which are targeted...

s and starfish.
Sand dollars, like all members of the order Clypeasteroida, possess a rigid skeleton

Skeleton

The skeleton is the body part that forms the supporting structure of an organism. There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body.In a figurative sense, skeleton can...

 known as a test

Test (biology)

A test is a term used to refer to the shell of sea urchins, and also the shell of certain microorganisms, such as testate foraminifera and testate amoebae....

. The test consists of calcium carbonate plates arranged in a fivefold radial pattern.