322 



THE BIOLOGY OF MARINE ANIMALS 



dependent upon balanced stimulation of two photoreceptors. The com- 

 pound eyes of Hemimysis are spherical and are located on movable stalks ; 

 consequently it has binocular vision over 360° and is able to orientate 

 while proceeding towards and away from the light (7, 49, 163). 



Other forms of taxes are the dorsal-light reaction and the light-compass 

 reaction. The dorsal-light reaction is found in many animals which norm- 

 ally swim horizontally with the dorsal surface uppermost, such as Charybdea 

 (Scyphomedusa), Tomopteris (Polychaeta) and Palaemon (Malacostraca). 



n 



-Xt3B>'" 



5&^ 



a 



l*-0'6mrn-»| 



Fig. 8.13. Orientation of the Larva of Arenieola in a Light Beam 



Arrows indicate direction of the light rays. The larva swims towards the light source 

 in a spiral course. At d the light is changed from m to n, and the animal aligns itself to 

 the new light source. (Lower right) enlarged view of the head, showing paired ocelli. 

 (From Mast (105).) 



In species with statocysts, both statoreception and photoreception are 

 involved in regulating the response. The mechanism of the dorsal light re- 

 action of Palaemon is depicted in Fig. 8.14. Palaemonis a prawn that possesses 

 statocysts; in normal posture its dorsal surface is kept uppermost, what- 

 ever the direction of incident light. When an intact animal is held obliquely 

 it makes pushing movements with its lower limbs that would tend to 

 return it to the normal position. Quite otherwise is the behaviour of an 

 animal which has been deprived of its statocysts. When turned over on its 

 side so that its back is towards a source of lateral light, it maintains a 

 symmetrical position without attempting to resume an upright posture. 



