| A. Alveoli neither within nor without the central capsule. | ||||
| Nucleus spherical (sometimes ellipsoidal), not branched, | ||||
| 1. Actissa. | ||||
| B. Numerous large alveoli within the central capsule (not in the calymma). | ||||
| Nucleus spherical, | ||||
| 2. Thalassolampe. | ||||
| Nucleus branched or covered with radial sacs, | ||||
| 3. Thalassopila. | ||||
| C. Numerous large alveoli without the central capsule, within the jelly-veil or calymma. | ||||
| Nucleus spherical, | ||||
| 4. Thalassicolla. | ||||
| Nucleus branched, or covered with radial sacs, | ||||
| 5. Thalassophysa. | ||||
Genus 1. Actissa,[[8]] n. gen.
Definition.—Thalassicollida with simple spherical nucleus, without any alveoli (either within or outside the central capsule).
The genus Actissa is the most simple and typical form of all Radiolaria, and may be regarded as the common ancestral form of the whole class. The spherical body is composed of a simple spherical capsule and a concentric spherical calymma or jelly-envelope. Neither the former nor the latter contains alveoli. The central capsule possesses a strong membrane perforated by small pores, and contains in the intracapsular sarcode numerous small pellucid vacuoles, and in its middle a simple, concentric, spherical nucleus (often with some nucleoli); sometimes also one or more oil-globules. The extracapsularium forms a soft, voluminous, structureless calymma or enveloping jelly-sphere, perforated by the numberless, fine pseudopodia, which radiate outwards from the matrix or the thin granulated sarcode-layer, surrounding the capsule. Often (but not constantly) xanthellæ or yellow cells are scattered in it. Actissa differs from the following skeletonless genera in the absence of all alveoli; it has neither intracapsular alveoli (like Thalassolampe) nor extracapsular alveoli (like Thalassicolla). The first observed species of this genus is that which I found in 1866 at the Canary Islands, Actissa prototypus; the second is that which Hertwig accurately described in 1879, from the Mediterranean (Messina), Actissa primordialis; the third I observed in 1881 at Ceylon, frequent and sporiparous, Actissa princeps. A fourth species (Actissa radiata) exhibited a distinct radial segmentation of the capsule-contents. These four species are quite spherical. Six other species, occurring in different preparations from the Challenger, are distinguished by modifications of the spherical capsule-form and may represent three different subgenera (or, perhaps better, genera?)—Actiprunum ellipsoidal, Actidiscus lenticular, Actilarcus lentelliptical; perhaps these are the ancestral forms of the three sections: Prunoidea, Discoidea, Larcoidea.
Subgenus 1. Procyttarium, Haeckel, 1879.
Procyttarium, Haeckel, Natürl. Schöpfungsgeschichte, ed. vii. p. 705.
Definition.—Central capsule spherical.
1. Actissa princeps, n. sp. (Pl. [1], fig. 1).
Central capsule spherical, colourless or a little reddish, transparent, with a thick double-edged membrane. Nucleus central, spherical, one-third as broad as the central capsule, containing a single, central, glossy nucleolus. Protoplasm finely granulated, without oil-globules, with numerous clear spherical vacuoles of equal size and at equal distances; the superficial layer of protoplasm (immediately below the membrane) radially striated (fig. 1). In some older specimens the nucleus was divided into numerous small nuclei (fig. 1a), which by further division gave the nuclei of the flagellated spores (fig. 1b); each spore with a very thin lateral flagellum (fig. 1c). Jelly-like calymma twice as broad as the enclosed capsule, without xanthellæ or yellow cells, pierced by innumerable, very thin and long, undivided pseudopodia, which arise from the sarcode-matrix on the outside of the membrane (six to eight times longer than shown in fig. 1).
Dimensions.—Diameter of the central capsule 0.1 to 0.12, of the nucleus 0.03 to 0.04, of the jelly calymma 0.2 to 0.3.