In addition, there are many detailed parallels in the participation of genes during the induction and specification of different cell fates in the two visual organs (Friedrich 2006b).
It even regulates some aspects of development in nematodes which lack visual organs altogether (Hodin 2000; Piatigorsky 2008).
Although they demonstrate how direct adaptive evolution could play a major role in the evolution of visual organs, their emphasis on linear transformation tends to obscure the complexity of the actual process.
It is even possible that Pax6 has been independently recruited for a role in guiding visual organ development in different lineages (i.e., by parallel evolution) because it is one of the few regulatory genes expressed "at the right place at the right time" during development that could be co-opted into this role in each lineage (Hodin 2000).
Tardigrades typically possess a pair of simple, ocellus-like eyes (Kristensen 1983; Dewel et al. 1993; Greven 2007)—a situation which is similar to that in onychophorans but different from that in arthropods, which show two types of visual organs: The median ocelli and the compound eyes (fig. 1 A C).
In conclusion, I suggest that the last common ancestor of arthropods bore only one pair of ocellus-like visual organs that were modified in several arthropod lineages.
Scarce and controversial information on visual organs and their innervation in Onychophora currently do not allow a thorough comparison with Euarthropoda.
This shows visual organs as they occur in contemporary species none is ancestral to another, and this does not necessarily reflect a historical series of steps in the evolution of complex eyes (see Fig. 10).
Balance in the human body, the stability of its characteristic upright postural position, is maintained, controlled, and monitored by a complex system, consisting of the vestibular organs, the visual organs, and the organs of deep perception, touch, and pressure.
BEST VISUAL EFFECTS Or to be more precise, best effects by a visual organ.
As patient TJ had no visual field defect, he could be tested in both visual fields in this experiment.
