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The linkage complex is composed of two subcomplexes, (1) a spindle
shaped midpiece (mp) that rests in a groove of the ER
(er) on 5 slats that pass through the ER membrane and have long
filaments (f) that splay from the spindle tips to reach through
the epiplasm to the inner alveolar membrane and (2) a number of rails
(R) that parallel the midpiece on its two sides. The rails also
have components that pass through the ER membrane. Though
characterized structurally in 1973 their function is still a matter of
untested speculation. The most reasonable guess is that they are sites
of calcium flux from the ER to the myoneme (M) and/or from the
alveoli which are known to store calcium. They are probably also
involved in pumping calcium out of the myoneme for storage in the ER.
Removal of calcium to less that 10-7M results in
relaxation, e.g., the extension of the spasmoneme or myonemal bands,
so that the zooid is extended away from the substrate and the oral
region is again opened out for feeding. ri, ribosomes. Drawn by
R. Allen. Published in J. Cell Biol. 56:559-579, 1973 and J.
Protozool. 20:25-36, 1973.
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