306 



THE BIOLOGY OF MARINE ANIMALS 



their diurnal migrations and rhythmic activities. In animals with highly 

 organized nervous centres, it influences other still more complex behaviour 



patterns. 

 Photoreception and photosensitivity in animals can be studied at several 



(d) 



Fig. 8.1. Receptors in the Skin of Chord ates 

 (a) Sensory cells in the skin of Amphioxus; (b) end knobs, with sense cells and nerve 

 supply, in the skin of the sturgeon (Acipenser); (c) free nerve endings in the epithelium 

 of the fin of the sturgeon; (d) terminal body in the pectoral fin of Squatina. (All after 

 Bolk et ah, 1934, from various authors.) 



levels. Usually some overt reaction or physiological response of the animal 

 constitutes the signal for photosensitivity. In any animal it is usually poss- 

 ible to select some one or few behavioural responses which can be utilized 

 as a sign of the existence and the degree of photosensitivity. These often 

 involve locomotory changes or movement of some structure. At a higher 



