Busch, 1851
Tiny, aberrant, information scant; anatomical homologies obscure. Body greatly flexible, elongate-suboctohedral when swimming, shape like two 4-sided near-pyramids base-to-base; length reported to ca. 15 mm; equatorial groove demarcating oral portion from aboral, oral noted as 2-5 times length of aboral. Four long ridges on edges along whole length, continuous across equatorial groove; prominence varied, bearing nematocyst tracts, with other tracts and scattered nematocysts betweeen ridges. Marginal lobes (? lappets) 16, 2 pairs between adjacent ribs; digitate distally, 8 final branches per quadrant. Sense organs (rhopalia?) 8, statocysts without ocelli, 2 on each side, one between each pair of marginal lobes. No tentacles or mouth-arms. Mouth opening single, simple; gastrovascular system a continuous cavity leading to one canal within the length of each longitudinal rib. Gonads 4, equatorial, each with 4 sausage-shaped lobes aligned longitudinally, two directed orally, two aborally.
Coloration: recorded as whitish to bluish-white.
Remark: the nominal species Tetraplatia chuni Calgren, 1909, only recorded from the South Atlantic (Rees and White, 1957b), has been separated on lacking external longitudinal ridges.
(After Russell, 1970; Pagès et al., 1992).