Dimensions.—Main axis of the ellipsoid 0.12, equatorial axis 0.09; pores 0.005, bars 0.004; size of the equatorial protuberances and of the polar tubes 0.02.
Habitat.—Pacific, central area, Station 265, depth 2900 fathoms.
Family XIII. Spongurida, Haeckel (Pl. [48], figs. 6, 7).
Spongurida, Haeckel, 1862, Monogr. d. Radiol., p. 447 (sensu emendato).
Definition.—Prunoidea with spongy ellipsoidal or cylindrical shell, composed wholly or partially of a spongy framework, without equatorial stricture, with or without an enclosed medullary shell.
The family Spongurida comprises, in the sense here restricted, all those Prunoidea in which the ellipsoidal or cylindrical shell is composed wholly or partially of an irregular siliceous framework, not of simple lattice-work. It contains two subfamilies, differing in the absence or presence of a latticed medullary shell in the middle of the central capsule; in the Spongellipsida it is absent, in the Spongodruppida present; the former are most nearly related to the Ellipsida, the latter to the Druppulida, the difference consisting only in the spongy structure of the cortical shell.
In my Monograph (1862, p. 447) the family Spongurida had a much wider extent, comprising also a number of Sphæroidea and Discoidea, agreeing in the spongy structure of the shell. Here we restrict the definition to those spongy Sphærellaria in which the central capsule and the enclosing spongy cortical shell are ellipsoidal or cylindrical, therefore each transverse section is a circle, and each meridional section an ellipse, as in all Prunoidea.
The Cortical Shell in all Spongurida is composed of a delicate framework of irregularly branched and interwoven siliceous threads; commonly this spongy structure is rather dense or compact, but sometimes also very loose. In the simplest form, Spongellipsis, the spongy cortical shell contains a large cavity, in which lies freely the central capsule. In Spongurus this cavity is completely distended by a spongy framework. In this case the solid spongy shell becomes often prolonged, and its original ellipsoidal form passes over into a cylindrical one (as in many Zygartida). Sometimes (particularly in Spongocore, Pl. [48], fig. 6) the cylinder becomes three-jointed by two more or less distinct annular constrictions. Spongocore is distinguished by an outer veil, a thin lattice-lamella, which envelops the spongy shell and is connected with it by radial beams.
The Medullary Shell, absent in the Spongellipsida, is constant in the second subfamily Spongodruppida. It is either a simple latticed shell (Spongodruppa) or double, composed of two concentric latticed shells (Spongoliva); its form is either spherical or ellipsoidal. It lies in the middle of the central capsule, and is connected by radial beams (perforating the membrane of the latter) with the enveloping spongy cortical shell.
In many Spongurida the surface of the shell is armed with radial spines, and in some genera (Spongoprunum, Spongatractus, &c., Pl. [17], fig. 12), on both poles of the main axis, are developed two strong opposite polar spines, as in many other Prunoidea.