Sphærozoida, Haeckel, 1862, Monogr. d. Radiol., p. 521.
Definition.—Beloidea socialia.
The family Sphærozoida comprises all associated or colony-forming Radiolaria, which are provided with an imperfect skeleton, composed of numerous solid needles or spicula, scattered around the central capsule in the calymma. The structure and form of this skeleton is quite the same as in the preceding solitary Thalassosphærida, but on the other hand, the structure and form of the colonies and of the included numerous central capsules is the same as in the skeletonless Collozoida.
The oldest well-known form of Sphærozoida is the common cosmopolitan Sphærozoum punctatum, probably first observed in 1834 by Meyen, and called Sphærozoum fuscum, afterwards more accurately described by Huxley in 1851.
Other forms were afterwards described by Müller and by myself in 1862.[[21]] Further investigations have shown me that some species of this family are among the most common Radiolaria, and occur in astonishing numbers on the surface of all warmer seas. But the number of species is comparatively small, and their distinction is very difficult, as all the different forms are very variable and connected by intermediate forms—a truly "transformistic" group.
The only character sufficient for the constitution of genera in this transformistic group is found in the form and composition of the spicula; the very variable form of the jelly-calymma and the enclosed central capsule being without value for this purpose. But also the form of the spicula is very variable, and not always constant. In some species the particular form of the spicula is transmitted by constant heredity, whilst in others it is very inconstant, even in one and the same individual. (Compare the remarks on variability in the general introduction.)
As the number of various forms is rather great, it seems to be advisable to distinguish the three following genera.
Synopsis of the Genera of Sphærozoida.
| A. Spicula all of one kind, simple or needle-shaped, | 12. Belonozoum. |
| B. Spicula all of one kind, branched or radiate, or geminate, | 13. Sphærozoum. |
| C. Spicula of two more different kinds, partly simple, partly branched, | 14. Rhaphidozoum. |