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Sand dollars and their relatives (clypeasteroids, cassiduloids) have a bilaterally symmetrical test composed of TEN double columns of plates, five interambulacral columns (labelled 1-5) and five ambulacral columns (labelled I-V). Ambulacral plates are pierced by single or double pores for the tube-feet. All five ambulacra are generally identical and on the aboral surface the pore-pairs form a very distinctive petaloid pattern.
There are two major openings in the test, the peristome and periproct. The peristome houses the mouth and is situated on the lower surface, either centrally or towards the anterior border. This is identifiable as the point towards which all five ambulacra converge, and is usually rather small and either circular or pentagonal in outline. In life a grill of spines projects across the peristome. The periproct houses the anal opening and is variable in position. In some species the periproct is positioned above the ambitus and can be lodged in a distinct groove, the anal sulcus. In others it may open on a vertical truncate face, while in many clypeasteroids it is on the oral surface. It is always surrounded by plates of interambulacrum 5.
At the apex of the test, at the point of origin of the ambulacral zones, lies the apical disc. This is composed of a small number of plates that are the first to form at metamorphosis. It is here that the openings to the gonads emerge. The plating arrangement of the apical disc provides a number of important taxonomic characters.
The adapical portions of ambulacra are variously developed into petals, specialised zones of enlarged pore-pairs that support respiratory tube-feet. The shape and relative development of petals is fairly uniform throughout the group, although the detailed pattern can be important for generic differentiation. In most groups the ambulacra are composed of simple plates. However, in certain clypeasteroids the petals may contain alternately large and small plates. Whether ambulacral pores are single or double below the petals is also important.
The arrangement of pores is highly variable in this group. In cassiduloids there are bands of enlarged pores close to the mouth that form phyllodes. Phyllodes house the feeding tube-feet, and their detailed arrangement is a taxonomically important character. Clypeasteroids differ from cassiduloids in having large numbers of tube-feet piercing each ambulacral plate. 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 perforate 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. The perignathic girdle may be made up of single interambulacral processes, or paired ambulacral processes. Cassiduloids have neither a lantern nor a perignathic girdle as adults, although a lantern is present in early development and is later resorbed.
In clypeasteroids the upper and lower surfaces of the test are connected by an often extensive series of buttresses and pillars. These are purely structural and reinforce the test.