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The anus of sand dollars is located at the back rather than at the top as in most urchins, with many more bilateral features appearing in some species. These result from the adaptation of sand dollars, in the course of their evolution , from creatures that originally lived their lives on top of the seabed ( epibenthos ) to creatures that burrow ...
The anus is located in this lunule. [2] [3] [4] The aboral surface is shallowly domed, the highest point being at the anterior petal, while the oral surface is flat. The body surface is covered with small spines which give it a velvety appearance. The colour of this sand dollar varies but is usually some shade of yellowish-brown or pale brown ...
Heliophora orbicularis, also known as the West African Sand Dollar, is a small sand dollar in to the family Rotulidae, and the only species in the genus Heliophora. It, and other members of Rotulidae have been found in West African marine strata from the Late Miocene onward. Like the related Rotula , it is still extant.
The mouth is at the base of the animal and the anus at the top; the lower surface is described as "oral" and the upper surface as "aboral". [a] [2] Several sea urchins, however, including the sand dollars, are oval in shape, with distinct front and rear ends, giving them a degree of bilateral symmetry.
The Gnathostomata are a superorder of sea urchins, including the familiar sand dollars. Gnathostomatans are irregular in shape, but unlike other irregular sea urchins, possess a feeding lantern. The mouth is located in the centre of the lower surface, as it is in most other sea urchins, but the anus is found to one side of the upper surface ...
The tests (shells) of these sand dollars are round, flat and disc-like, typically measuring 3 inches (7.6 cm) in diameter. The growth rate for this animal is between 3.5 to 6 mm/yr in the latter 5 years of their lifespan, and the lifespan is typically around 8 years. [2]
A few sea urchins and one species of sand dollar carry their eggs in cavities, or near their anus, holding them in place with their spines. [58] Some sea cucumbers use their buccal tentacles to transfer their eggs to their underside or back, where they are retained.
Dendraster excentricus, also known as the eccentric sand dollar, sea-cake, biscuit-urchin, western sand dollar, or Pacific sand dollar, is a species of sand dollar in the family Dendrasteridae. It is a flattened, burrowing sea urchin found in the north-eastern Pacific Ocean from Alaska to Baja California .