Knobbly sea stars are also dorsal-ventrally thicker than the common sea star. Adult knobbly sea stars are generally bright orange with black bumps on top of the sea star, as compared to the brown or gray common sea star. From knobbly sea star ( Protoreaster nodosus ) It can be distinguished from the similarly large knobbly sea star by the general appearance. typicus with the flat sucker-like disc 16 17. maculata are also pointed like the Astropecten spp., differing from A. maculata usually possesses eight arms, but can have any number from six to nine, with lengths from 120mm to 200mm compared to the usual five arms which do not usually exceeds 120mm in A. Luidia maculata at Changi Beach (Image by Samuel Chan, 2015) The terms aboral and oral refer to the side opposite the mouth and the side with the mouth respectively, commonly used in describing echinoderms. The parts of the sea star are labelled in the preserved specimen below, where the colour has been through preservation in ethanol. It possesses spines thought to aid it in burial into sand 9. Specimens with 4 to 7 arms have also been seen, though much rarer than the normal pentaramous ones 8. It usually possesses varying brown to gray colouration, with some variation of patterns on the arms. The common sea star resembles a typical large sea star with five arms and radial symmetry.
Together, this likely means that Archaster typicus was one of the main types of sea star. The species epithet typicus comes form the same Latin word meaning "of or pertaining to a type" 7. The origins of the scientific name, or its etymology, likely lies in the combination of Latin words arch meaning "chief" and aster meaning "star" 7. Sand-sifting sea star, while providing some description of it, can also refer to a variety of sea stars from the genus Astropecten 6. The name 'common sea star' can refer to a variety of species across the world, including the common starfish of the Atlantic, Asterias rubens 5 (For why we use scientific names, see this Guardian article 4 ). The name 'common sea star' is a classic example of why vernacular names cause ambiguity. Archaster typicus on exhibit at the Lee Kong Chian Natural History Museum (Image by Samuel Chan, 2015)
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