Thus, although isogamy consists in typical cases of a union of naked motile gametes by a fusion which begins at the beaked ends, and results in the formation of an immotile spherical zygote surrounded by a cell-wall, in Leptosira it is noticeable that the fusion begins at the blunt end; in a species of Chlamydomonas the two gametes are each included in a cell-wall before fusion; and in many cases the zygote retains for some time its motility with the double number of cilia.
The asexual cells are immotile spores arising in fours in sporangia from superficial cells of the thallus.
Until the recent discovery by Williams of motility, by means of a single cilium, of the antherozoids of Dictyota and Taonia, they were believed to be immotile bodies, like the male cells of red seaweeds.
The cause of this luminosity is Micrococcus phosphorens, an immotile round, or almost round organism.
While many forms are fixed to the substratum, others are free, being in this condition either motile or immotile.