Suborder PRUNOIDEA, Haeckel, 1883 (Pls. [13]-[17], [39], [40]).
Definition.—Spumellaria with an ellipsoidal or cylindrical central capsule, prolonged into one axis (sometimes articulate by annular transverse strictures); with an ellipsoidal or cylindrical, fenestrated siliceous shell (often articulate by annular strictures), invariably prolonged into one axis. Fundamental form monaxon, usually with the poles of the prolonged dimensive main axis equal.
The suborder Prunoidea comprises those Spumellaria in which the fenestrated spherical shell appears prolonged into one axis. The geometric fundamental form of the shell, which in the Sphæroidea was a sphere, in this case therefore becomes an ellipsoid, and whilst in the former all axes originally have the same value (Homaxonia), here one main axis is constantly larger than all other axes (Monaxonia). Usually both poles of this main axis are equal (Haplopola); but in some genera both poles become different (Diplopola).
In the Sphæroidea all planes going through the centre of the shell are circular, whereas in the Prunoidea only those planes are circular which are perpendicular to the main axis; all other planes going through the centre are elliptical; the largest of these are the meridian planes, in which is situated the main axis. Commonly all meridian planes are equal, as no transverse axes (or cross axes) are differentiated.
In my Monograph (1862) only very few forms of Prunoidea, such as Didymocyrtis and Spongurus, are described, and the greater part of them are distributed under different genera of Sphæroidea, such as Haliomma and Actinomma. In my Prodromus (1881) I separated them as the family Zygartida (p. [462]). But it seems now much more convenient to restrict this term to a particular family and to give a wider extension to the whole suborder under the name Prunoidea (called after the characteristic ellipsoidal form of a plum, or Prunus, with its stone).
The suborder Prunoidea comprises seven different families, of which the Ellipsida constitutes the simplest and the probable common ancestral group. In all Ellipsida the fenestrated shell is simple, and never composed of concentric or twin shells. In their primitive genus Cenellipsis, the whole shell is geometrically nothing more than a simple ellipsoid (Pl. [39], figs. 1, 2). By development of radial spines it passes into Ellipsidium, by development of spongy framework into Spongellipsis. In the greater part of this family large spines are developed on both poles of the main axis of the shell, but sometimes instead of these solid spines, two opposite hollow fenestrated tubes are developed (Pipettella, Pl. [39], fig. 6).
The second family of Prunoidea, the Druppulida, is much richer in different forms than the simple Ellipsida. In this case the ellipsoidal shell is composed of two or three (rarely more) concentric shells. Constantly one or two of these fenestrated shells are enclosed in the central capsule, and may therefore be called "medullary shells"; and one or two (rarely more) lie outside the central capsule, "cortical shells." The inner medullary shells (one or two) are either spherical or ellipsoidal; the outer cortical shells (one or two, rarely more) are always ellipsoidal. All concentric shells are connected by radial beams. In the simplest form of the subfamily, Druppula (Pl. [39], fig. 3), one medullary shell is connected with one cortical shell. By duplication of the medullary shell arises Prunulum (Pl. [39], fig. 4), and by duplication or multiplication of the cortical shell Cromyodruppa (Pl. [15], figs. 1-4) is formed. In by far the greater portion of this subfamily large spines are developed on both poles of the main axis of the cortical shell (Pls. [16], [17]), but sometimes also instead of these solid spines two opposite hollow fenestrated tubes are developed (Pipetta, Pl. [39], figs. 7, 8).
A third family of Prunoidea, closely allied to the two preceding families, is the Spongurida, in which we include all Prunoidea with an ellipsoidal or cylindrical, unjointed shell, in which the lattice-work of the cortical shell is transformed into an irregular, siliceous framework. In the simplest form, Spongellipsis, the simple lattice-shell of Cenellipsis is substituted by an external spongy envelope. In other cases (Spongurus and allied genera) the whole cavity of this external spongy shell is distended with a fine spongy framework. The subfamily of Spongodruppida is distinguished by the possession of a simple or double latticed medullary shell; this lies in the midst of the central capsule, and is connected by radial beams (perforating its membrane) with the enveloping spongy cortical shell. The surface of the latter may bear either radial spines, or two opposite strong polar spines, at the poles of the main axis (Pl. [17], fig. 12).
Closely allied to the Ellipsida and Druppulida are two other families of the Prunoidea, the Artiscida and Cyphinida, which differ from the former by a circular constriction in the equatorial plane of the ellipsoidal shell; and in this way assume a characteristic twin form, like a figure of eight. In the Artiscida the shell is simple (as in the Ellipsida), whereas in the Cyphinida it is composed of two or more concentric shells (as in the Druppulida). The simplest form of the Artiscida is Artiscus (Pl. [39], fig. 9), differing from Cenellipsis in the ring-shaped, equatorial constriction. In other Artiscida polar appendages are developed on both poles of the main axis, either in the form of solid, strong spines (Stylartus), or hollow fenestrated tubes (Cannartus, Pl. [39], fig. 10).