A baranch of the water vascular system that runs up the middle of each ambulacrum.
Muscles connecting the pyramids to the perignathic girdle and used to pull the lantern into the test.
Calcareous elements (usually five) that are arranged in a circle to form the frame of the suckered disc of tube-feet.
A kind of pedicellaria with strongly curved blades that touch only at their distal tips. Not associated with poison glands.
An anterior region of the test that is prolonged and projecting. The picture shows the cassiduloid Archiacia in lateral profile.
Part of the Aristotle's lantern. A bone-shaped ossicle that forms a brace between adjacent pyramids. It articulates with the epiphyses.