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Echinometra mathaei grows to a test diameter of about 5 centimetres (2.0 in). The colour is quite variable but the test is usually a dark colour. The spines are sometimes green and purple with purple tips or entirely green with purple tips but this sea urchin can be distinguished from other species by a characteristic pale ring at the base of each spine.
The name urchin is an old word for hedgehog, which sea urchins resemble; they have archaically been called sea hedgehogs. [6] [7] The name is derived from the Old French herichun, from Latin ericius ('hedgehog'). [8] Like other echinoderms, sea urchin early larvae have bilateral symmetry, [9] but they develop five-fold symmetry as they mature ...
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 .
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, rather than being central. The members of the group are adapted for burrowing in soft-bottomed marine environments.
Sand dollars (also known as sea cookies or snapper biscuits in New Zealand and Brazil, or pansy shells in South Africa) are species of flat, burrowing sea urchins belonging to the order Clypeasteroida. Some species within the order, not quite as flat, are known as sea biscuits. Sand dollars can also be called "sand cakes" or "cake urchins". [2]
These cells may be light-sensitive, causing many echinoderms to change appearance completely as night falls. The reaction can happen quickly: the sea urchin Centrostephanus longispinus changes colour in just fifty minutes when exposed to light. [23] One characteristic of most echinoderms is a special kind of tissue known as catch connective tissue.
Toxopneustes pileolus, commonly known as the flower urchin, is a widespread and commonly encountered species of sea urchin from the Indo-West Pacific. It is considered highly dangerous, as it is capable of delivering extremely painful and medically significant stings when touched.
Echinocardium australe is a sea urchin in the family, Loveniidae, first described by John Edward Gray in 1851, [1] [2] from specimens collected in Port Jackson and Tasmania. [2] It is a synonym of Echinocardium cordatum .