Definition.—Thalassicollida without intracapsular alveoles, but with large roundish or globular alveoles within the extracapsular calymma. Nucleus in the centre of the capsule simple spherical, not branched.
The genus Thalassicolla was proposed by Huxley in 1851, for a certain number of different voluminous jelly-like Radiolaria, which he had observed living during his voyage in the "Rattlesnake" through the tropical seas, and of which he gives an excellent description—the first accurate observations on living Radiolaria. Johannes Müller afterwards removed from this genus the social genera Sphærozoum and Collosphæra (formerly Thalassicolla punctata), and retained as type of the genus Thalassicolla nucleata. In 1862 in my Monograph I added two other species, Thalassicolla pelagica and Thalassicolla zanclea, and later (1870) Thalassicolla sanguinolenta. Now I think it better to separate the last two species as a new genus, Thalassophysa, characterised by the papillate or branched nucleus, and to retain in Thalassicolla only those forms with simple spherical nucleus. For both genera the extracapsular, voluminous, spherical calymma or jelly-envelope, with numerous large alveoles, is characteristic. The membrane of the central capsule in Thalassicolla is now structureless (subgenus Thalassicollarium, with three species), now characterised by a peculiar structure, prominent ridges on the inside of the membrane, which form a network with polygonal plates, resembling an epithelium (Pl. [1], fig. 5b; subgenus Thalassicollidium, with four species). Of the seven species here described, two are cosmopolitan, widely distributed, and common; one is Mediterranean, one Atlantic, and three Pacific.
Subgenus 1. Thalassicollarium, Haeckel.
Definition.—Membrane of the central capsule structureless, only perforated by innumerable very small radial pores.
1. Thalassicolla pellucida, n. sp.
Spherical body very soft, transparent, clear and colourless, without any pigment. Central capsule soft, hyaline, with a thin, structureless, not areolated membrane. Diameter of the central capsule about three times that of the nucleus, one-fourth to one-sixth that of the jelly-envelope. Nucleus delicate, transparent, with one single central nucleolus, about one-third its diameter. Protoplasm of the central capsule contains only small, pellucid, densely packed globules (vacuoles?), no oil-globules. Extracapsular body quite transparent, without pigment or oil-globules, only composed of the large alveoles imbedded in the jelly-cover, and of the fine protoplasmic network between them. No xanthellæ.
Dimensions.—Diameter of the central capsule 0.8 to 1.2, of the nucleus 0.3 to 0.4, of the calymma 3 to 6 mm.
Habitat.—Cosmopolitan, Canary Islands, Haeckel; Cape, Australia, Pacific, Challenger; surface.
2. Thalassicolla spumida, n. sp.
Spherical body nearly transparent, yellowish, without dark pigment. Central capsule pellucid, with a thick, structureless, not areolated membrane. Diameter of the central capsule about twice that of the nucleus, one-sixth to one-eighth that of the jelly-cover. Nucleus delicate, somewhat opaque, with numerous small nucleoli. Protoplasm of the central capsule contains small pellucid globules (vacuoles?), and immediately under its membrane (on its inside) one single layer of large, dark, refractive oil-globules. Extracapsular body very voluminous, foamy, with innumerable alveoles in the jelly, and many xanthellæ between them.