SENSORY ORGANS AND RECEPTION 321 



Oriented Locomotory Responses. In the simplest type of locomotory 

 response to light, an animal seeks or avoids an illuminated region by a 

 kind of trial and error activity classified as photokinesis. In a non- 

 directional light gradient animals displaying photokinesis congregate in 

 light or dark regions. When there is a tendency to shun the light the animals 

 may move more rapidly or change the direction of movement more often 

 in illuminated areas, with the result that they remain longer or come to 

 rest in dark regions. This kind of response is shown, inter alia, by turbellar- 

 ians. 



Animals which possess suitably organized photoreceptors can make use 

 of the directional properties of light. When placed in a horizontal light 

 beam they move directly towards or away from the light source according 

 to whether they are photopositive or photonegative. The simplest type of 

 reaction to directed light, known as klinotaxis, is shown by animals which 

 make regular swaying movements, and which are able in consequence to 

 make successive temporal comparisons of light intensity. Examples are 

 Euglena and the planktonic larvae of many benthic animals. The photo- 

 receptors in these animals are simple stigmata or ocelli partially shielded 

 by pigment and so organized that the retina is stimulated by light coming 

 from some particular direction with reference to the axis of the animal's 

 body. 



The larvae of Arenicola, like those of many other polychaetes, are 

 strongly photopositive after hatching, and aggregate at the lighted surface 

 of the water by means of klinotaxis. When swimming, the larva rotates on a 

 longitudinal axis and, if it be laterally illuminated, each eye is directed 

 alternately towards and away from the light (Fig. 8.13). When one of the 

 eyes is directed towards the light, the body contracts and the head turns 

 in that direction; this soon results in orientation and the animal continues 

 to swim towards the light source (104, 105). 



Animals with well-developed photoreceptors orientate directly to a 

 light source in a straight path without pendular movements. In one form 

 of response the animal is able to orient itself through achieving balanced 

 and equal stimulation of symmetrically disposed paired photoreceptors 

 (tropo-taxis). Such responses are widespread in annelids, gastropods, etc. 

 When confronted with two light sources it proceeds at some angle between 

 them, depending on the relative intensity and stimulating power of the 

 two lights. 



More complex are those responses in which the animal moves directly 

 towards a light source without the necessity of balanced stimulation of 

 two receptors (telo-taxis). In this response the animal orients directly to 

 one of two lights. When Hemimysis, for example, is exposed to a single 

 light, it swims to and fro in line with the beam of light. But when an ad- 

 ditional light is arranged with its beam at right angles to the first, some of 

 the mysids remains in the first light beam, while others cross over and move 

 along the second beam. The mysid is thus capable of selecting by some 

 central process one of the two light beams for orientation, and is not 



M.A. — n 



