The pores associated with these tube-feet may be regularly arranged or irregularly scattered. The shape of the first interambulacral plates surrounding the peristome is significant. 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 .