Marine animals usually with a pentamerous symmetry, though some early forms have no radial symmetry, and some later echino-derms develop a superimposed bilateral symmetry. Skeleton composed of calcite plates.
subphylum HOMALOZOA (Cambrian to Devonian)
The carpoids; echinoderms without radial symmetry.
subphylum CRINOZOA (Cambrian to Present)
Benthic echinoderms, with a clear pentamerous symmetry. This group includes the crinoids (which are usually attached by a stalk to the sea floor). Other crinozoans (e.g. some cystoids and Mas-toids) have no stalks. Arms usually provide food to the mouth.
subphylum ASTEROZOA (Ordovician to Present)
The starfish, typically with five arms. Some primitive forms are suspension feeders, but most are carnivorous. Asteroids have arms which merge towards the centre, while ophiuroids have long arms which are distinct extensions from a central disc.
subphylum ECHINOZOA (Cambrian to Present)
Free-living echinoderms, which include the Lower Cambrian Heli-coplacoidea (which have only one ambulacrum), the Edrioaster-oidea, the leathery Holothuroidea, and the sea urchins (Echinoi-dea). Many advanced echinoids develop a bilateral symmetry, which is especially associated with the burrowing deposit-feeding heart urchins.
PHYLUM CONODONTOPHORA (Cambrian to Triassic)
Small tooth-like and plate-like structures of calcium phosphate (conodonts) belonging to an unknown group of extinct marine-animals.
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