SENSORY ORGANS AND RECEPTION 



323 



Again, if the light is coming from the side while the animal is held in its 

 normal position, it makes pushing movements with its legs on the side 

 opposite the light; these movements would tend to turn the animal on 

 its side. In the absence of statocysts, therefore, Palaemon orientates itself 

 solely by phototaxis. Another prawn, Processa, which lacks statocysts, 



"^ -¥ V 



a ^Ea lb *TJb 



IHa 



IYa 



Mb 



rvb 



— y 



— > 



^^£^--— , 



—> 



Ya Yb 



Fig. 8.14. Orientation Responses of a Prawn under the 

 Influence of Light and Gravity 



I-IV, animals suspended freely ; V, resting on the bottom ; a, intact animals ; b, animals 

 deprived of statocysts. Broken arrows, left and above, indicate direction of incident light. 

 Curved arrows indicate sense of turning. Small broken arrows in figures indicate 

 direction of thrust of leg. When the animal is suspended freely and held obliquely, the 

 legs on the lower side make pushing movements which would return the animal to its 

 normal position (back uppermost) if it were free to move (\a-Wa). Lateral illumination 

 does not interfere with this statocyst reaction. When an animal with statocysts removed 

 is held suspended, while light is coming from the side, it makes pushing movements with 

 its legs on the side opposite the light (III6). When resting on the bottom and exposed to 

 lateral light, an animal lacking statocysts compromises by inclining the body at an 

 angle of about 45°. (After Alverdes, 1926.) 



shows photic responses resembling those of Palaemon when deprived oi' 

 statocysts. 



There are some invertebrates which show orientation to some fixed 

 locus, and a faculty of this kind assumes importance in an animal that 

 has some fixed home to which it returns after each excursion. Cues from 

 several sensory fields may be used in achieving spatial orientation, in- 

 cluding a photic response known as the light-compass reaction. In this 

 reaction the animal need not orientate directly towards the source of 

 light, but instead is able to maintain some definite angle between the light 

 source and its direction of motion. Although best known in terrestrial 



