Siliceous Shells of Protozoa

Abstract

IT is of course well known that some Protozoa (as Radiolaria and Heliozoa) have the hard parts siliceous, while others (as Foraminifera) have them calcareous. Sponges show analogous groups. In 1911,1 when discussing the freshwater genus Quadrulella (Rhizopoda), I remarked: “In his recently published (1911) paper on the Rhizopods of the British Antarctic Expedition, Penard calls attention to the curious fact that while the square plates of Q. symmetrica are siliceous, those of Q. irregularis (Archer) are calcareous, and will dissolve in hot concentrated sulphuric acid. He therefore inclines to agree with Awerinzew that the two animals are really generically distinct, and represent a case of convergent evolution.” There has just been published a very interesting paper by Heron-Allen and Earland2 in which they describe a new genus (Miliammina) of Foraminifera with a siliceous test, represented by five species in the Antarctic. But, in 1914, Chapman dealt with one of these species, calling it a new variety arenacea of the northern Miliolina oblonga (Montagu). The new form is said to be a siliceous ‘isomorph’ of the porcellanous M. oblonga. The question naturally arises, whether in such cases we do have ‘convergent evolution’, as I postulated in 1911, or divergent evolution, the form of the shell being ancestral, the material of it changed. Heron-Allen and Earland are a little ambiguous about the type species of their new genus. I now formally designate as such Miliammina arenacea (Chapman), which they, for no apparent reason, call M. oblonga (Chapman).