The Echinoid Directory

Phormosoma Thomson, 1872, p. 493

Diagnostic Features
  • Test large with flexible plating throughout.
  • Apical disc monocyclic with plating often reduced and genital pore surrounded by a number of small platelets.
  • Ambulacra trigeminate, with large central element bearing a primary tubercle and two highly reduced adradial elements (demiplates) housing pore-pairs. On the oral surface the demiplates abut the adradial suture but aborally they are occluded from both perradial and adradial suture.
  • Ambulacral zones about two-thirds the width of interambulacral zones at the ambitus.
  • Pore-pairs forming an irregular adradial column.
  • Interambulacral plates wide, with rows of subequal primary tubercles.
  • Primary tubercles on oral surface large and with distinctly sunken areoles; tubercles perforate and non-crenulate.
  • Aboral tubercles irregularly arranged.
  • Pore-pairs on peristome forming two series in each zone.
  • Oral spines ending in a rounded point. Aboral spines in life ending in enlarged fleshy sacs.
  • Sphaeridia associated with both demiplates.
Recent, Indo-Pacific and Atlantic.
Name gender neuter
Phormosoma placenta Thomson, 1872, by original designation.
Species Included
  • P. bursarium A. Agassiz, 1881; Recent, Indo-Pacific.
  • P. placenta Wyville Thomson, 1872; Recent, Atlantic.
  • P. verticillatum Mortensen, 1904; Recent, Indian Ocean.
  • P. rigidum Agassiz, 1881; Recent, New Zealand.
Classification and/or Status

Euechinoidea; Echinothurioida; Phormosomatidae.



Distinguished from other echinothurioids other than Hemiphormosoma by the large, sunken areoles of its oral interambulacral tubercles. Hemiphormosoma has primary interambulacral tubercles present only close to the ambitus, with both adoral and adapical plates relatively naked.

Mortensen, T. 1935. A monograph of the Echinoidea. Volume 2, Bothriocidaroida, Melonechinoida, Lepidocentroida and Stirodonta. C.A. Reitzel, Copenhagen.