SENSORY ORGANS AND RECEPTION 



327 



methods. Briefly these involve study of the behaviour of the animal in 

 differently coloured lights, and measurement of the electrical activity of 

 photoreceptors (either retinal action potentials or optic nerve impulses), 

 when the retina is stimulated with light of selected wave-length and known 

 intensity. The spectral sensitivity curves obtained by these methods are 

 termed action spectra. 



Some spectral sensitivity curves for lower animals, based on behaviour 

 studies, are shown in Fig. 8.17. These are all marine species with the ex- 

 ception of the freshwater sunflsh Lepomis auritus. The data for Cerianthus 

 were obtained by measuring the reaction time for the heliotropic response 

 to light of different wave-lengths. For Mya, the reaction time for with- 

 drawal of the siphons was measured. Telotaxis in two differently coloured 



- 1-5 



30 



90 



120 



60 

 Time (minutes) 



Fig. 8.16. Course of Dark- adaptation in a Lamellibranch Mya 



AND AN ASCIDIAN ClOna 



The curves were established from reaction times to a standard flash after various 

 periods in the dark. Ordinates, reaction times in seconds: left, Ciona; right, Mya. 

 (Curves from Hecht (66), redrawn and smoothed.) 



light beams was used to determine the action spectrum of Palaemonetes 

 larvae. The behavioural response of Myxine which was utilized was 

 movement preparatory to swimming when the animal was illuminated. 

 For the sunfish, the visual rheotropic response was employed. Further 

 action spectra, for Limulus and Eledone, based on electrical responses of 

 retina and optic nerve, are shown in Fig. 8.18. Most of these curves have 

 the same general shape, but there are significant differences in wave-length 

 of maximal sensitivity, ranging from 490 to 550 m/u (58, 67, 108a, 111, 136, 

 159). 



Colour Vision. Colour is one of the attributes of an object which make 

 it visually recognizable. The fundamental discrimination between wave- 

 lengths takes place in the retina, specifically in the cones of the vertebrate 

 eye, but colour vision occurs only when wave-length differences are 

 recognized in the c.n.s. To demonstrate colour vision in animals necessi- 

 tates tedious training experiments (139). 



