animals. As the flatworm gets older, it relies on photosynthate from the internal algae, and its digestive organs degenerate so that it is not able to feed like a normal animal. The Convoluta, however, needs a more varied diet than just that of algal photosynthate and begins after a time to feed upon the algae – to kill the geese that laid the golden eggs – so that the algae gradually disappear, the flatworm presenting the strange appearance of a green head and a white tail. Finally the flatworm dies of starvation, but not before it has laid a large number of eggs.

Charophyceae

This is the line of algal evolution that led to the development of land plants. The motile cells of the advanced members of the class are similar to the flagellated male gametes of the bryophytes and vascular cryptogams. The motile cells of the Charophyceae are asymmetrical and have two laterally or subapically inserted flagella. The microtubular root system contains a multilayered structure that is associated with a broad microtubular root and a second,

smaller, microtubular root. Rhizoplasts are not present. The mitotic spindle is persistent during cytokinesis, and cell division occurs by means of a phragmoplast. No eyespots occur. Sexual reproduction results in the formation of a dormant zygote. Meiosis occurs when the zygote germinates. Glycolate is broken down by glycolate oxidase, whereas urea is broken down by urease. The algae in the class are predominantly freshwater algae.

The unicellular alga Mesostigma is thought to be the closest extant relative to the “ancestral green flagellate” and represents the most primitive alga in the Charophyceae (Marin and Melkonian, 1999; Lemieux et al., 2000). Mesostigma viride (Fig. 5.15) is a freshwater biflagellate alga with flagella arising from a depression in the cell (Manton and Ettl, 1965). Each flagella is anchored by two microtubular roots, with one root having an associated multilayered structure (Melkonian, 1989). The single chloroplast contains pyrenoids, an eyespot, and unique pigments (Yoshii et al., 2003). The cell is covered by three layers of scales. An inner layer of five-sided scales, a middle layer of oval-shaped scales, and an outer layer of basket-shaped scales.