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and the trichocysts of dinoflagellates are compact, linear bodies containing a highly organized latticed framework of carbohydrates. When these bodies are released through discrete pores through the surface of the cell, the extrusomes become hydrated and rapidly extend in length as spear-like threads (Fig. 4.2D and I) (Hausmann, 1978). Although the origin and function of extrusomes in both groups are not clear, they probably play a role in escape responses, defense, and capturing prey cells.

Benthic euglenids and dinoflagellates, in particular, adhere to substrates and are capable of gliding motility using 2 heterodynamic flagella equipped with flagellar hairs (or mastigonemes). In both groups, the recurrent flagellum sits within a groove on the ventral surface of the cell and is oriented backward. Euglenids and dinoflagellates also possess cytoskeletal elements (called “paraxial/paraflagellar rods,” which run in parallel to the 9 + 2 microtubular axonemes within each flagellum) that are not found in any other group of eukaryotes. A major difference between euglenids and dinoflagellates, however, is the structure, orientation, and motility of the anterior flagellum. The anterior paraxial rod in euglenids is oriented on the ventral side of the axoneme, is stiff and held straight in front of the cell; the paraxial rod functions with the flagellar hairs to produce gliding forces (Saito et al., 2003). By contrast, the anterior flagellum of dinoflagellates forms a transverse loop or spiral around the circumference of the cell and usually sits within a transverse groove called the cingulum (Fig. 4.1). The coiled transverse flagellum bears hairs and a flagellar membrane that connects it to the base of the cingulum, and this entire apparatus is capable of producing forces on the surrounding medium that tend to spin the cell around its longitudinal axis.

Many free-living euglenids and dinoflagellates engulf prey organisms using sophisticated feeding apparatuses positioned on the ventral side of the cell. Although the evolution of these apparatuses is a shared fea-

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