Figure 5.

Most motile unicellular organisms display a pattern of motility involving rotation while progressing rectilinearly. This is the case of the single cell Chlamydomonas (a) and of the colonial multicellular Volvox (c). If Chlamydomonas would form colonies and would keep the same flagellar motion, the colony would not move (b). The Volvox colony progesses along its antero-posterior axis which corresponds to the antero-posterior axis of individual cells only for those cells positioned in the most anterior part of the colony. All the other cells cannot progress along their own antero-posterior axis, nor can they rotate around their axis. In (d) the cellular polarity of each cell (a cell close to the equator of the colony is depicted) will be such that the cellular antero-posterior axis will differ for each position, but the anti side will always be towards the colonial anterior pole, thus respecting a planar polarity, as judged by the positions of the eyespots (adapted from Hoops [150]).