The Echinoid Directory

Hapalosoma Mortensen, 1903, p. 64

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. Only becoming adradial in position close to the apex and peristome. Pore-pairs arranged in three series on the oral surface. Primary tubercles absent from perradial and interradial zones, leaving broad naked bands. Ambulacra about half the width of interambulacra at the ambitus. Interambulacral plates with prominent membranous gaps on both upper and lower surfaces. No primary interambulacral tubercles interradially and no ambulacral primary tubercles on oral surface. Shallow buccal notches. Only ambulacral plates extending over the peristome, with pore-pairs arranged uniserially. Spines ending in expanded cone-shaped hyaline hoof. Sphaeridia on only the inner accessory plate in each triad.
Distribution	Recent, Indonesia and Japan.
Name gender	neuter
Type	Asthenosoma pellucidum A. Agassiz, 1879, by original designation.
Species Included	H. pellucidum (Agassiz, 1879); Recent, Indonesia. H. gemmiferum Mortensen, 1934; Recent, Japan.
Classification and/or Status	Euechinoidea; Echinothurioida; Echinothuriidae; Echinothuriinae Monophyletic
Remarks	Differs from Araeosoma in having primary tubercles absent from perradial and interradial zones, leaving broad naked bands and in having highly derived dactylous pedicellariae. It also has ambulacral plates continuing onto the peristome as a single series in each half ambulacrum in contrast to the double series seen in Calveriosoma and Araeosoma. However, the species included in this genus are relatively small. 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. Shigei, M. 1986. The sea urchins of Sagami Bay. Maruzen Co., Tokyo.