Meso forms aldaric acids

Both are optically active. Had the C-4 hydroxyl group been to the left, one of the aldaric acids would have been a meso form. 

These two hexitols readily form mono- and di-anhydrides. The majority of sugar alcohols readily form acetal and ketal derivatives . When the terminal groups in aldoses are made alike, loss of asymmetry of the molecules often occurs the resulting aldaric acids and alditols, as for example, xylaric acid and galactitol, are optically inactive, non-resolvable meso forms. Fischer made use of this fact in his assignment of sterciochemical formulae to the monosaccharides. 

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