iv. is 



PHOTORECEPTORS OF LAMPREYS 



white objects and probably use both the eyes and the nose to find 

 their prey. In the ammocoete larva the paired eyes are buried below 

 the pigmented skin and the animal makes no movements when light 

 is shone on to this region. 



The optic tracts of adult lampreys end in the roof of the midbrain 

 (tectum opticum) which is a highly differentiated, stratified region. 

 Besides the optic fibres it receives also impulses from fibres ascending 

 from the spinal cord and others from the auditory and lateral line 



Fig. 74. Experiment to show behaviour of larval lampreys when illuminated. The 

 tank is left in total darkness and the larvae settle in all parts. When the light is 

 switched on those in the illuminated part begin to swim and continue to do so until 

 by chance they arrive in the darkened part, where they settle down. (From Young.) 



centres. The midbrain is therefore undoubtedly one of the most 

 important parts of the brain in lampreys, though nothing is known in 

 detail of its functions. Its cells control movements of the animal, by 

 means of fibres that run to make connexion with the dendrites of the 

 large Midler's cells, whose axons pass down the spinal cord; other 

 fibres from the tectum opticum reach to various parts of the brain, 

 and it is probable that its activities are closely correlated with those of 

 many other regions. 



15. Skin photoreceptors 



Like many lower vertebrates the lamprey has light-sensitive cells in 

 the skin, as well as those in the eyes. These receptors are abundant in 

 the tail and if a light is shone on to this region the animal rapidly 

 moves away (Figs. 74 and 76). If the spinal cord is cut just behind the 

 head and a light then shone on to the tail, the head will be seen to 

 move. This suggests that the impulses are carried forwards by means 

 of the lateral line nerves, which is confirmed by the fact that if these 

 latter are sectioned, leaving the spinal cord intact, then no movements 

 follow when the tail is illuminated. This sensitivity of the lateral line 



