How many cells does the smallest animal have?
Rotifers are microscopic protostomes with around 1000 cells apiece.  This website claims the smallest rotifers have less than 100 cells, but I couldn't confirm this.  Most rotifers are eutelic — within a given species, each adult individual has the same number of cells (Encyclopeia of Life: Rotifers).


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Edit: Myxozoans might be even smaller.  These are microcsopic parasites that have been classified as cnidarians based on molecular studies.  They have complicated life cycles including a single-celled stage.


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