**2.3 Motility**

The motility of these cells is due to two asymmetric flagella and its cellular form. Both flagellums have rows of mastigonemes similar to those of stramenopiles [32]. The longest flagellum directed to the front has two rows of bipartite hairs, and the shortest flagellum directed to the back only has one, they can be observed by electron microscopy, and structure variations have been described [28]. The bipartite hairs are composed of a tubular attached to the axoneme, it terminates with a single non-tubular filament in the longest flagellum, but in the case of the shortest, it ends with two unequal terminal filaments [9, 20, 28]. In addition, delicate seven-sided scales measuring 140–170 nm in diameter are commonly attached to the hairs [33]. Other structures that are part of the mechanism for motility are the rhizostyle and a compound rootlet system. The rhizostyle is a peculiar microtubular flagellar root that originates near one basal body and extends toward the posterior extreme of the cell; in some species until the nucleus, without a physical connection, but in others, just to the first third of the cell and has a wing-shaped lamellar projection [29, 34]. The cryptophytes have a phototaxis response that is mediated by a two-rhodopsinbased photosensory mechanism, similar to what is observed in green flagellated algae *Chlamydomonas reinhardtii* [35]. The structure of this mechanism includes an integral membrane protein, with a seven transmembrane alpha-helices covalently bonded to the retinal chromophore to make a channel structure, it is the anion channelrhodopsin, which is light-gated, and initially discovered in chlorophyte algae, which serve as photoreceptors to guide phototactic orientation [36, 37], this structure has been utilized in optogenetic applications [38].
