386 



Comparative Animal Physiology 



Euglena moves directly toward or away from it, depending on the intensity. 

 When the direction of illumination is changed suddenly, the animal re- 

 orients, as shown in Figure 102. This type of orientation is called klinotaxis, 

 a directional orientation made possible by means of regular deviations in 

 movement (pendular or spiral), and involves comparison of intensities at 

 sucessive points in time. The coelenterate Hydra behaves in this way, as do 

 the post-trochophore larva of Arenicola and the tadpole larva of the ascidian 



Sensory cells 



Lens 



Pigment cells 



Accessory cells 



Fig. 103. Diagrammatic representation of photosensitive organs. A, Section through 

 the ocellus of Sarsia. B, Transverse section of the eye of Planaria mactdata. C, Section 

 through eye of Sepia. D, Section through eye of Pecten. Redrawn from Hyman,™ 

 Taliaferro'"^' and Borradaile and Potts.'"'" 



Amaroucium. Earthworms, under certain conditions, exhibit a pendular 

 movement resulting in orientation, as do the maggot larvae of several species 

 of flies." ^^ 



With the advent of two or more receptors, a peripheral mechanism exists 

 which may permit direct orientation to light. In Planaria maculata, which 

 possesses two cup-eyes, the orientation of the animal is in that direction 

 which results in equal stimulation of both eyes. This type of orientation is 

 called tropotaxis, and involves the comparison of intensities on two receptors 

 simultaneously. Planaria, deprived of their eyes and illuminated, show an 



