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Rhizoplegma boreale (Cleve, 1899)

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Hexadoras borealis:
Primordial shell: irregularly spherical, 0.03 to 0.04mm in diameter with irregular, rounded or polygonal pores, 2 to 3 on the radius, and thin bars. Spines six, exceptionally more, strong, with triangular apophyses in the middle.

Outer shell: a rounded or octahedral, more or less intricate network of anastomosing, siliceous threads, issuing from the proximal edges of the spines. Spines usually six (rarely as in Rhizoplegma 8 to 10) strong, three-sided, and slightly spirally twisted, with elegantly aculeate, winged edges.
Cleve 1899
Rhizoplegma boreale:
Easily recognized by the characteristic interwoven pyramids around the main spines. Haeckel depicts similar pyramids for Rhizoplegma lychnosphęra. The inside shell is irregular, not quite round; sometimes resembling a cube in shape, sometimes it is more like an octahedron. The pores are uneven, irregular, polygonal, averaging about 10µm. The intermediate walls between the pores are not broad, differently developed, but never having the distinct broader corners which correspond to the rounded lumen of the pore. A few small, short, needle-shaped byspines are found here and there on the beams, but not regularly in the corners.

The main spines are long and strong, often 8 in number (according to Haeckel's system answering to an inner cube), although also often only 6 (answering to an octahedron) or 7. A larger number may sometimes be found. The main spines have 3 strongly developed edges which narrow off toowards both ends. On these edges there are transverse branches diverging at right angels and these, together with the corresponding ones on the other edges of the same spine, form a very variable number of verticils of threes.

The spongy, loose and very irregularly constructed network, which forms the outer shell, arises from the lowest 2-3 verticils on the main spines, but has the appearance of being lengthened out a good way up along the radial spines, on account of the thin connecting beams, which unite the different verticils parallel to the direction of the spine.

Also on the outside of the outer spongy shell (network), there are some few scattered short and fine, needle-shaped byspines. The ocean forms seem generally to have 6 radial spines. There seems to be a slight difference between the regular forms which have 6 radial spines, and those which are more frequently found in the coast water and which have about 8. The former seem to have a smaller inner shell with distinct byspines. I have, however, not discovered any definite difference so as to make it necessary to divide them into two or more species.

Young forms are rather unrecognizable, as both the interwoven pyramids and the spongy outer shell are absent. The construction of this species resembles greatly that of Rhizoplegma radicatum HCK and Rhizoplegma lychnosphęra HCK. The inner, fine, transverse branches on the radial spines in the space between the outer, spongy shell and the inner shell are, however, wanting. These transverse branches are by Haeckel the characteristic of the subgenus Rhizoplegmidium. According to Haeckel's system, it might perhaps be a question as to whether or not our species should be classed as belonging to the genus Lychnosphęra, because of the byspines on the inner shell. As these, however, are small and few in number, and perhaps not even always present, this would not be recommendable, and still more so as such a distinction between the genera seems to be unnatural. Haeckel's genus Hexadoras has radial spines without side branches, for which reason I still mean that the above species finds a more natural place among the closely allied forms of the genus Rhizoplegma, notwithstanding that there undoubtedly often occur forms with 6 main spines.

Belongs to the most common radiolaria with us and is not rare in deep water samples, although never numerous.

Distribution: On the west coast of Norway rather frequent, although rare in fully developed condition. Known from the sea west and south of Spitzbergen (Cleve, 1899) and from a few places in the Norwegian Ocean.
Jųrgensen 1905
Rhizoplegma (?) boreale:
Skeleton consists of small inner sphere, radial needles and fine-lattice skeletal mass. Inner sphere of irregular shape, with irregularly disposed pores (5-7 on half equator). From it radiate 8-10 stout trihedral radial needles. Processes arise at right angles from each facet of needle; from these apophyses develop, uniting with each other and forming irregular lattices of basic skeletal mass. Between inner sphere and this lattice is small space. Radiating needles have processes in form of small barbs along entire length, but lattice is formed at first from beyond lattice. Lattice not of uniform thickness, being more developed around radial needles, where its layer is thicker, and becoming thinner in spaces between needles.

Dimensions: diameter of inner sphere 30-40µm, space between internal sphere and lattice 15-20µm, diameter of chamber formed 55-70µm, thickness of layer of lattice mass 30-50µm, diameter of lattice space 10-15µm, width of radial needles 8-10µm.
Petrushevskaya 1967


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