Unicellular green algae of the order Zygnematales. Like Spirogyra, they have an elaborate chloroplast. The cells of desmids are characteristically split into two halves joined by a narrow neck, each half being a mirror image of the other. Some form filaments comprising long chains of cells. The outer wall of the cell is patterned with various protuberances and covered with a mucilaginous sheath, which is thought to play a role in the cell’s slow gliding movement. Desmids are found mainly in fresh water and belong to the clade streptophytes (charophytes) that includes land plants.