Pennate diatoms are bilaterally symmetric, not radially symmetric. On each valve
there are openings in the form of slits along the surface, termed raphes (Fig. 2.6).
Typically, the shell is elongate parallel to the raphes. Cytoplasm streams along the
raphes, generating cell movement. The cells always move along solid surfaces, and
pelagic forms move only against each other in colonies. The best candidate
mechanism for this locomotion (Edgar & Pickett-Heaps 1984) is based on
ultrastructural analysis. Strands of mucus excreted through the raphe attach to the
substrate while maintaining a connection to actin filaments through a complex of
membrane-associated proteins (Edgar & Pickett-Heaps 1983). Actin is a contractile
protein, important in practically all motile functions from cytoplasmic streaming to
muscle contraction. Myosin is one element of the protein complex, and its movement
against the actin filaments produces a rearward translocation of a membrane protein
and its attached mucilage strand along the length of the raphe (Fig. 2.7a & b). At the
rear of the cell the strands are broken off by the forward traction generated by the
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