Definition of the family Isanthidae
Carlgren (1938, p. 59) defined the family Isanthidae as “ Nynantheae with basilar muscles. Sphincter mesogloeal. Mesenteries divisible into macro- and microcnemes. No acontia.” In his catalogue, Carlgren (1949, p. 76) changed the family definition to “Thenaria (Mesomyaria) with well developed mesogloeal sphincter. Mesenteries divisible into macro- and microcnemes. No acontia. Retractors of mesenteries very strong, strongly restricted (reniform) to almost circumscribed.” Due to the weak sphincter of Eltaninactis infundibulum, Dunn (1983, p. 58) proposed to alter the first sentence of the generic description to “Thenaria (Mesomyaria) with mesogloeal sphincter...” instead of “….with well developed mesogleal sphincter…”. Uncertainty remains over the affiliation of the genus Eltaninactis to the family Isanthidae, as noted in Sanamyan’s (2001) description of E. psammophorum.
Type species: Isanthus capensis Carlgren, 1938 (Fig. 6)
Definition of the genus Paraisanthus Sanamyan and Sanamyan, 1998:
Isanthidae with well-developed pedal disc. Column smooth. Margin distinct. Sphincter mesogloeal, well developed. Radial muscles of oral disc and longitudinal muscles of tentacles ectodermal. Two siphonoglyphs, two pairs of directives. Six pairs of macrocnemes. More mesenteries distally than proximally, at least in adults.
Cnidom: spirocysts, p-mastigophores, basitrichs, heterotrichs, holotrichs.
Type species: Paraisanthus tamarae Sanamyan and Sanamyan, 1998