SENSORY ORGANS AND RECEPTION 



307 



level, where ocular structures for form-vision exist, experiments have been 

 devised for evaluating the importance of shape and pattern in behavioural 

 responses (arthropods, cephalopods, vertebrates). These experiments fre- 

 quently utilize the learning capacity of animals and their ability to form 

 conditioned reflexes. From a purely physiological approach, photorecep- 

 tion can be studied by recording the electrical signs of ocular activity. 

 This method reveals the photosensitive capabilities of the eye and the 

 mechanisms involved in the conversion of photic stimuli into nerve impulses. 

 Before proceeding to a consideration of the behaviour of animals under 

 photostimulation and the physiology of photoreceptors, we shall examine 



primary sense cell 



large reticular gland 



ciliary rootlet cone / goblet gland 



ir>dffferertt,"*siipportrng* J limiting membrane bipolar ganglion cell 

 Ciliated epithelial cell . nerve fibre latjer _j 



Fig. 8.2. Diagrammatic View of Skin of an Enteropneust 

 (Saccoglossas) (from Bullock, 1946.) 



briefly the range of structure encountered in the light receptors of marine 

 animals. 



Structure of Photoreceptors 



All gradations of complexity exist in the structure of photoreceptors 

 among invertebrates and lower vertebrates, from simple pigmented spots 

 bearing a photosensitive surface to the complex camera eyes of cephalopods 

 and vertebrates. In some cases sufficient experimental data are available 

 to permit the characteristics of photoreception to be correlated with the 

 structure of a given photoreceptor. Comparative accounts of photo- 

 receptors in lower animals are available in Plate (121), Hesse (70) and 

 Warden et ah (147). Quantitative data may be found in Kahmann (77). 

 The comparative physiology of sensory perception, including photo- 

 reception, is treated by Prosser et ah (123). 



Simple Photoreceptors 



The general integument of many lower animals is sensitive to light, 

 although light organs as such may be wanting. By studying the behaviour 



