Sea urchin

Sea urchins do not have arms and so it is confusing that they are related to sea stars. However, it is not this physical feature that unites the phylum it is by their tube feet that relates them. Sea urchins have five rows of tube feet. These tube feet can be suckered or unsuckered and are useful in attachment, locomotion, feeding, and gas exchange. Sea urchins are spherical in shape and covered in spines averaging 1-3 cm in length. If touched, the spines will quickly converge toward the point that has been touched. Urchins feed mostly on algae but will also feast on other invertebrates.