PHOTOTAXIS (Neo-Lat., from Gk.I5Cis, plus, light + rcits, taxis, arrangement, from to arrange). (1) ln animals, control of the direction of locomotion by light. Like the uni cellular plants, the Protozoa as well as higher animals may migrate in a direction determined by that of the rays of light. According as the mi gration is toward or from the source of light, we can distinguish positive and negative phototaxis. The flagellate Infusoria (C'hilonnonas and En gleim ) will move toward the light, crowding to gether on the illuminated side of the vessel. Ainceba has been observed by Davenport to he strongly phototactie, showing that such respon siveness is a general property of protoplasm, the result of chemical changes produced by light. Ciliate infusorians are not markedly phototactic. Cells containing such different kinds of pigment as the chlorophyll of plants and the retinal pig ment of the eyes of arthropods respond to the action of light in a similar way, this response being an adaptive one. A striking example of phototaxis in pigmented cells (chromatophores) is described by Keller. He has discovered that the dark color of the (illuminated) skin is due to the rich branching at the base of the epider mis of black pigment cells lying deep in the eutis. In the dark, the pigment granules stream out of the branches into the cell-body, but the branches themselves are undisturbed. So long as the black pigment has this central position, the skin appears whitish. The light, on the contrary, causes the pigment, which is probably carried passively in the „plasma, to move centrifugally. (Sec PIGMENT.) Hydra was observed by Trembley to wander toward the light, and this is of advan tage to the animal because many of the Ento inostraca on which it feeds are also phototactic.
So also is the starfish. The phenomenon is especially marked in bilateral animals, such as planarians, annelids, crustaceans, flies and many other insects, mollusks, and vertebrates. As Davenport states, animals which liNe in shady places or in the dark are negatively phototactic, i.e. shun the light, while those living in the light are positively phototactic. The house-fly is well known to be phototaetic, but its maggot shuns the light. Butterflies, says Davenport, are at tuned to a high intensity of light, moths to a low intensity, so that bright sunlight, which calls forth the one, causes the other to retreat. On the
other hand, a light like that of a candle, so weals as not to stimulate a butterfly, produces a marked response in the moth. The males and fe males of ants have been observed while mating in the air to be strongly phototactic. but after that period they show themselves neutral (Loeb). The larva.. (nauplii) of barnacles as well as other pelagic animals rise to the surface of,the sea dur ing the night, but descend before the strong, sun light. Temperature also affects phototaxis: a low temperature causes several of the normal re sponses, while under a high temperature it is ac celerated: also Loeb has found that a concen trated medium, as when the water is rendered more salt, acts as the lower temperature. It is thought that, while in the retina of the eye the protoplasm is specialized for perceiving light, there is sonic evidence that in the eyeless ani mals the whole surface of the body contains such light-perceiving substances. This is well known to be the case with the earthworms, and perhaps with the oyster, Pholas, VIM), etc. The peleeypod Psammobia, time blind Proteus of eaves, and Triton cristatus when blinded. are irritated by rays of light, especially the blue rays, falling on the skin. Consult Davenport, Ex/merino-0g/ Morphology, part i. (New York. 1S9; 1. which contains a full bibliography. Compare Trmetsm.
(2) In plants, the sensitiveness to illumina tion. This may be fundamentally the same as heliotropism (q.v.). The effect of light upon the organism may be to accelerate or retard the movements of the motor organs on the illuminated side, this causing the body to be rotated until both sides are equally illuminated (see CIIEMOTAXIS and CIIEMOTROPISM ). when any progress will then necessarily be toward or away from the source of light. Organisms which ap proach the source of light are said to be posi tively phototactic, those Nvhicli recede from it negatively so. The same organism may show successively both forms of response in light of different. intensity. For insta nee, Englena viridis is positively phototactic in weak light, but nega tively so in strong.