In some cassiduloids these are developed as bourrelets and can project strongly as peg-like structures. Spines are always short and stubby and are rather uniform in structure. They are hollow and without a cortex. Tubercles are correspondingly small, dense and often slightly sunken. There is very little spine and tubercle differentiation in comparison to spatangoids.
Tubercles are preforate and crenulate.
In most sand-dollars there are five grooves radiating from the peristome, and these may branch many times to form an intricate pattern of food grooves . These are lined with tube-feet and are used to transport food particles to the central mouth. Another prominent feature of some sand dollars are slit-like perforations termed lunules . Sand dollars have both a lantern and a perignathic girdle. The lantern is unusual in being highly flattened and lacking compasses.