Histology of the pedicellariae of the Sand dollar, Dendraster excentricus (Echinodermata)

The pedicellariae in Dendraster excentricus are the bidentate type with a density of 80/cm². There are two distinct size groups; the larger ones measure 0.6 mm long while the small ones are only 0.14 mm. One interesting feature is the fluid of highly sulphated acid mucopolysaccharide in the lumen of the neck. This fluid functions as a hydroskeleton while the flexor muscle operates on it for complicated movement of the neck. The possibility that this fluid is also toxic and so acts as a defence mechanism is discussed. All the muscles involved in the movement of pedicellariae are smooth muscle fibres.