Where the shape comes from
The mother 母 with her two dots run together into a single bar — a line struck across the graph, and a prohibition written over it. Kangxi files the mother herself under this heading, along with everything built on her: 每, the 每 of 每天, and 毒. The mother-shape is also the phonetic inside 海, which is why the sea and every 每 look alike on the page.
汉字 filed under 毋
No published character on HSK Guide is filed under this radical yet. 汉字库.