my proposal for sessile obbligato is completely sessile, that's the meaning
Mussels and Clams (Bivalvia) - The Living World of Molluscs
www.molluscs.at › bivalvia
So it is quite easy to stop a muscle from closing its shell, but very hard to open it with the mollusc holding tight from inside. .... Swimming mussels, as well as completely sessile mussels, often have a largely reduced foot.

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Note added at 1 hr (2017-08-24 21:38:08 GMT)
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For some weeks the larvae swim the sea together with other plankton. From the fertilised egg hatches a larva that is called a trochophora, because it bears an equatorial ciliar belt and a terminal ciliar crest. After the first metamorphosis the trochophora transforms into a larval stage, that possesses two ciliated sail-like protrusions and thus is called veliger (sail bearer). As well as the trochophora the veliger is a planktontic larval stage. After a further metamorphosis the veliger changes into a bivalve small juvenile oyster. That clings on a suitable rock with its byssus threads and sticks its shell to he rock with a self produced cement. In their further life the oyster will not leave its place any more.
http://www.molluscs.at

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Note added at 2 hrs (2017-08-24 21:54:24 GMT)
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more grits for totally sessile