Crossword puzzle clue "they're usually seen in rows"

One who rows, rows, rows the boat

They're usually seen in rows

Web concerns ... and based on six familiar names hidden in rows 1, 4, 12 and 15 of this puzzle grid, what the black squares in those rows symbolize

Rows of rows

They're usually seen with sandals

They get into rows at school

They come three rows to a package

They usually leave two when they're done