The theca of most species closely encloses the cell body, but can become separated due to irregularities in cell shape. Theca are usually smooth, but some species have developed distinct creases that cross perpendicularly near the cell apex, giving the cell the appearance of being divided into four longitudinal sections. Some species also have a twisted cell posterior that is mirrored in the thecal wall produced. The only perforation of the theca is where the flagella emerge, which is termed the apical depression. The pore is a long slit with thecal extensions that border it, extending upwards to protect the flagella base. In cells that have lost their flagella the thecal extensions grow to cover the slit, but they do not fuse; non-motile cells of "Tetraselmis" have flagellar stubs that remain within the thecal compartment. Additionally, many species have thick curly hairs anchored to the cell wall near the flagellar base. The hairs are cross striated and have a spiral pattern, which is similar to flagellar hairs, but they are thicker and longer than flagellar hairs. Scale production begins during prophase, and the scales are brought to the cell membrane promptly after completion of cytokinesis and the two daughter cells are separated. Fusion of the scales happens externally and begins near the posterior end, extending forwards to the anterior end in which the flagellar slit forms last.