ture, the details of these ultrastructural systems are quite distinctive. For instance, there are a few chief components present in most of the predatory species of euglenids described so far, namely “rods” and “vanes.” Two feeding rods oriented longitudinally within the cell are composed of microtubules and amorphous proteinaceous material. These stiff elements provide structural support for gripping and internalizing prey cells and work in concert with 4–5 membranous vanes that are usually reinforced with additional microtubules (Leander et al., 2007). The vanes originate from the rods, form the inside core of the feeding apparatus, and create space within the apparatus by opening up in a pinwheel-like fashion; the same mechanism can cause the apparatus to protrude from the cell when feeding. By contrast, the diversity and complexity of feeding apparatuses in dinoflagellates probably reflect independent origins in different lineages within the group. The feeding apparatus in dinoflagellates can be simple pockets that unzip when prey is drawn into the cell, dynamic siphons that suck out the cytoplasm of prey cells in a straw-like fashion or expansive veils that completely envelop large filamentous prey and fold it methodically into manageable packets small enough to ingest. Different kinds of feeding apparatuses are often associated with different kinds of photoreceptive eyespots and ocelloids, suggesting that in some dinoflagellates, photoreceptors are adaptations for detecting and capturing photosynthetic prey. Some predatory euglenids with a rod-and-vane feeding apparatus also possess a photoreceptor system, as a putative stigma and photosensory swelling (Leander et al., 2001), and this combination of features may serve the same basic function as in dinoflagellates.
Another convergent similarity between benthic euglenids and dinoflagellates is the tendency to reinforce their cell surfaces with robust proteinaceous layers beneath the plasma membrane (Fig. 4.2C and H). Euglenids possess a distinctive (and synapomorphic) pellicle consisting of discontinuous strips that run longitudinally or helically over the entire cell surface (Leander et al., 2007). The strips articulate along their lateral margins, and in many euglenids these zones facilitate sliding between strips that produce rhythmic deformations in cell shape, called “euglenoid movement.” Benthic dinoflagellates can also change their shape, especially after engulfing large and oddly shaped prey cells. The proteinaceous surface layer in dinoflagellates, called the “dinoflagellate pellicle” forms a continuous and flexible sheath beneath alveolar vesicles, which may in turn be filled with cellulosic material. Both the euglenid and the dinoflagellate pellicles comprise novel classes of proteins: articulins and epiplasmins (Bouck and Ngo, 1996). Although it is unclear whether these proteins represent an example of molecular convergence or distant homology, their presence in both euglenids and dinoflagellates underscores the striking similarities between these 2 very distantly related groups of eukaryotes.