Definition.—Astrosphærida with three concentric, spherical, lattice-shells, united by radial beams.
Genus 101. Actinomma,[[137]] Haeckel, 1862, Monogr. d. Radiol., p. 440.
Definition.—Astrosphærida with three concentric lattice-spheres and numerous simple radial spines of one kind.
The genus Actinomma is here restricted to those Sphæroidea which combine the possession of three concentric lattice-shells with numerous radial spines on the surface; the spines are all simple, being of one and the same kind. Commonly two of the three shells are intracapsular medullary shells, connected by radial beams (piercing the central capsule) with the outer, extracapsular, cortical shell. But in some species only one medullary shell is enclosed in the central capsule, whilst both other shells lie outside it. In such case the distance between these two cortical shells is much smaller than their distance from the simple internal medullary shell. These forms correspond more to aculeate Rhodosphæræ, whilst the others resemble aculeate Thecosphæræ.
Subgenus 1. Actinommantha, Haeckel.
Definition.—Pores of the cortical shell regular, of nearly equal size and similar form; spines on the entire surface (commonly one spine at each nodal-point).
1. Actinomma hexagonium, n. sp.
Cortical shell, as well as both medullary shells, very thin walled, with regular, hexagonal pores and thread-like bars between them. Pores of the outer shell twice as broad as those of the middle, and three times as broad as those of the inner shells. Radial proportion of the three spheres = 1 : 3 : 9; about twenty thin radial beams between them. At each nodal-point of the surface arises one bristle-shaped radial spine, half as long as the radius.
Dimensions.—Diameter of the outer shell 0.22, middle 0.07, inner 0.025; meshes of the cortical shell 0.01; length of the spines 0.05.
Habitat.—Central Pacific, Stations 270 to 274, surface.