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The World of Protozoa, Rotifera, Nematoda and Oligochaeta

Ref ID : 2040

Harald Netzel; [Structure and Ultrastructure of Arcella vulgaris var. multinucleata (Rhizopoda, Testacea)]. Arch.Protistenk.Bd. 117:219-245, 1975

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The theca of Arcella vulgaris var. multinucleata is composed of hollow chambers or alveoli arranged in a single layer. In radial section the alveoli are orthogonal in outline and 700-900 nm in height. In tangential section they are usually dodecagonal and are of such a size that they may be circumscribed by a circle 1 um in diameter. Short and long sides alternate around the edge of the dodecagon. The wall of each alveolus is apparently homogeneous and about 20 um broad except at the outer surface of the test, where it is slightly thicker (25 nm). The alveolar lumen is filled progressively from the wall inwards by fibrillar appositions and granular inclusions. In the cupola-like dorsal side of the shell the alveoli are usually arranged in a hexagonal pattern, whereas in the funnel-shaped ventral part the packing is often less regular. A theca may consist of from about 3.500 to 11.000 alveoli. In the region between three (or four) apposed alveoli the thecal wall is perforated by pore channels, which are about 150 nm in width. The section of the pore channel is that of a triangle with rounded vertices. The pore channels border on the short radial walls of adjacent alveoli. The outer aperture of the pore channels is slightly narrowed by a short projection of the external alveolar wall. These are from about 7.000 to 22.000 pore channels in a test. The cytoplasm contains numerous electron-dense, membrane-bounded droplet-shaped granules. The granules are 850-900 nm in length and have a maximum diameter of 500-650 nm. Apart from this external form polarity the granules also exhibit an internal structural polarity. They originate in a typical position relative to the dictyosome, which appears to be involved in their formation. The granules presumably contain keratinoid protein(s). They present the building material for construction of new thecae, and are therefore referred to as thecagenous granules.