Phormosoma placenta is a species of sea urchin in the order
Echinothurioida. It is a deep water species, seldom being found
at depths less than 500 metres (1,600 ft), and occurs on either
side of the Atlantic Ocean on the continental slope. Phormosoma
placenta is a yellowish-brown colour and can grow to a diameter
of 12 cm (5 in). The flexible test is dome shaped above and
flattened beneath. The plates from which the test is made
overlap each other and are bound together by a membranous
connection. Specimens removed from the water usually collapse
into disc shapes. The upper (aboral) surface has few primary
tubercles and spines but the lower (oral) surface is densely
covered in perforate tubercles from which slender, club-shaped
spines project, each one embedded in a membranous sac. These
spines articulate with the tubercles and are used to support the
animal and also in locomotion. Observations of live individuals
on the seabed show that the few spines on the aboral surface are
also enclosed in large membran