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Illustrated is the generalised internal and external features of a typical arbacia-type sea urchin. Several features such as gills and the pedicilaria were omitted as they are outside the scope of this image.
Sea urchins are globular, superficially pentamerically symmetrical echinoderms which inhabit marine environments world wide. Movement across the substrate can be made in one or both of two ways: The defensive spines can be moved independently and thus act as mobile stilts in the long spined species, or the hydraulically extended tube-feet can be employed for both mobility and anchorage.
These tube feet are part of an internal network of water filled tubes and bladders called the water vascular system. This network is connected to the seawater via a structure called the madreporite on the aboral (upper) surface: When the bladders along the radial canals contract, they inflate and thus extend the tube feet.
The sea urchin feeding apparatus is a five sided complex articulated structure called the Aristotles' Lantern. It consists of a series of interlinked plates and muscles which manoeuvre five continuously growing teeth, used to rasp away at algae, their predominant food source.
The nervous system is relatively simple, centred around a nerve ring inside the lantern from which radiate five nerves to innervate the radial water vascular system.