Sciurella Allman, 1883

Galea, Horia R., Maggioni, Davide & Di Camillo, Cristina G., 2021, Towards a better understanding of the genus Sciurella Allman, 1883 (Cnidaria: Hydrozoa: Plumulariidae): evidence from an integrative study, Zootaxa 5040 (1), pp. 1-32 : 4-5

Genus Sciurella Allman, 1883

Type species. Sciurella indivisa Allman, 1883 , by monotypy.

Diagnosis. Colonies erect, composed of multiple stems springing in clusters from the hydrorhiza. Stems thick but monosiphonic, either simple or forked, divided into somewhat regular internodes; coenosarc canaliculate. Stem internodes usually bearing four cladial apophyses, of which a pair is set close to the proximal node, while the other is found next to the distal node, the pairs forming an acute angle between them and giving the hydrocladia a bipinnate arrangement along the stem; the number of cladia per internode and their arrangement could be locally or fully altered, giving rise to stems whose appearances range from simple-pinnate to multi-verticillate. Cladial apophyses stubby, bearing a prominent adaxial mamelon, and a number of flanking nematothecae. Cladia unbranched, divided into elongate internodes, composed of usually one hydrotheca (but up to three possibly present) and its complement of three nematothecae. Hydrotheca long, almost tubular, fully adnate, with thick perisarc. Mesial nematotheca quite long, conical, borne on a distinct bulge of perisarc far below the hydrothecal base; lateral nematothecae comparatively shorter, sometimes becoming almost globular; all nematothecae bithalamic, wall of upper chamber with deep adaxial emargination. Gonothecae borne in pairs, one to each side of the cladial apophyses; gonothecae not known in all species but, when present, either stellate or urn-shaped, flattened dorso-ventrally and curving over the corresponding stem internode; as a rule, the gonothecae are provided with nematothecae (notably along their perimeter) whose coenosarc is composed of ramifications originating from a central blastostyle.

Remarks. Morphologically, the genus Sciurella is readily distinguished from Nemertesia Lamouroux, 1812 through the following features: 1) unlike in Nemertesia , its cladia are borne at different levels on the stem internodes, often adopting a bipinnate arrangement; 2) its hydrothecae are very deep and almost tubular, instead of being cup-shaped; 3) its gonothecae are armed with nematothecae, instead of being devoid of them; 4) the upper chamber of all nematothecae is distinctly emarginated on the adaxial side, instead of having an entire rim.