Some observations on the locomotion and feeding of the sand dollar, Dendraster excentricus (Eschscholtz)

Abstract

Dendraster excentricus was observed at Alki Point, Seattle, Washington, at a maximum density of 629 animals/m 2 during the summer of 1967. The animal usually remains half buried subvertically with the anterior end downwards at high tide and buries itself completely during low tide. There is no uniform orientation in relation to the tidal current; the animals lie randomly. Young animals of less than 15 mm in diameter move mainly by their suckered tube-feet; they can climb an aquarium wall. In the adult, primary oral spines are the major locomotor organs. Forward movement (progression) is the dominant pattern of locomotion, although a reversed movement is possible. The animal can bury itself within 15 min and right itself within 2 h. Young animals can bury and right themselves much faster than the adults. Suspended matter, composed mostly of diatoms, is first swept to the test surface and then to the food tracts by ciliary currents. In the food tracts the particles are caught in the mucus which is secreted by the mucus glands in the floor of the tracts. These mucus strings are then carried by the ambulacral tube-feet into the peristome where they are finally collected into the mouth by the teeth with the help of buccal tube-feet and oral spines.