This animal lacks the the five arms that are characteristic of the phylum but does posess the same five-part radial symmetry (Raven and Johnson 1999). It is generally about 5-10 cm in diameter when fully grown.
The shell has many small perforations that form a symmetrical petal-like design. The entire shell (or "test," since it is not truely a shell as it is covered by skin) is penetrated by many small, brown spines that give the shell a velvety appearance and enable the animal to move about. Since these creatures have found very effective hiding places in the sand, the spines no longer are needed for protection and have been modified ("Sand Dollar" 1997). The spines on the somewhat flattened underside of the animal allow it to burrow or to slowly creep through the sand. Fine, hair-like cilia cover the tiny spines. These cilia, in combination with a mucous coating, move food to the mouth opening which is in the center of the star shaped grooves on the underside of the animal (Page 2000). The opening for the anus is on the posterior edge of the test.
The holes in the test also allow for the tube feet of the characteristic water vascular system to protrude. These tube feet also aid in moving food to the mouth, as well as in burrowing. The test is divided into sections, each characterized by a calcerous plate that is fused to the next. The plates are either ambulacral or non-ambulacral, either with holes for the tube feet or without. This corresponds to the ambulacral grooves found in the sea stars (Anonymous 1998).
In addition to the small spines on the surface, there are small organs called pedicellariae. These organs are small jawlike structures that were once thought to be parasites.However, upon further observation, it was understood that these organs function in grooming the sand dollar and keeping would-be parasites away. This species posesses the smallest of these organs which functions mainly in grooming (Banister and Campbell 1985).
The calcareous test is what is commonly found washed up on the shore, sans the velvety spines and bleached by the sun.
Range mass: 10 to 25 g.
Other Physical Features: ectothermic ; heterothermic ; radial symmetry
Sexual Dimorphism: sexes alike