The Echinoid Directory

Hygrosoma Mortensen, 1903, p. 59

Diagnostic Features
  • Apical disc monocyclic, with ocular and genital plates forming an unbroken circle.
  • Ambulacra trigeminate, with a single large element reaching the perradial suture and two small demiplates positioned centrally and abutting one another adapically. On the oral surface the accessory plates become adradial in position.
  • Pore-pairs arranged as a single adradial series on the oral surface but may be triserial aborally.
  • Row of primary ambulacral tubercles on every principle plate.
  • Ambulacra about half the width of interambulacra at the ambitus.
  • Interambulacral plates with prominent membranous gaps.
  • A single primary interambulacral tubercle on every other plate only; rest of plate with just sparse granulation.
  • Buccal notches deep but rounded.
  • Ambulacral plates extending over the peristome, with plates arranged uniserially.
  • Spines with expanded cone-shaped hyaline hoof.
  • Sphaeridia on both accessory elements.
Recent; Indo-Pacific and Atlantic.
Name gender neuter
Phormosoma petersii A. Agassiz, 1880, by original designation.
Species Included
  • H. petersii (Agassiz, 1880); Recent, West Indies and East Atlantic.
  • H. hoplacantha (Wyville Thomson, 1877); Recent, Indo Pacific.
  • H. luculentum (Agassiz, 1879); Recent, Indo Pacific.
Classification and/or Status

Euechinoidea; Echinothurioida; Echinothuriidae.



Differs from Echinothuriinae in lacking membranous gaps between interambulacral plates and in having the three pore-pairs not forming three series in each half-ambulacrum. Differs from the fossil Echinothuria in having pore-pairs uniserial across the oral surface with accessory plates reaching the adradial suture.

Mooi et al (2004) provide a cladistic analysis of this genus and its species.

Mooi, R., Constable, H., Lockhart, S. & Pearse, J. 2004. Echinothurioid phylogeny and the phylogenetic significance of Kamptosoma (Echinoidea: Echinodermata). Deep Sea Research II 51, 1903-1919.

Mortensen, T. 1903. The Danish Ingolf-Expedition 1895-1896. Vol. 4, No. 2. Echinoidea, pt. 1, 198 pp., 21 pls. Bianco Luno, Copenhagen.

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