IV. IO 



104 VERTEBRATES WITHOUT JAWS 



It has been possible to find out something of the part that these 

 organs play in the life of the lamprey. When a bright spot of light is 

 directed upon the pineal region of a stationary ammocoete larva move- 

 ment is usually initiated, but only after illumination for many seconds. 



o.s.s. 



Fig. 



t.s.s., o.s.s 



66. Pineal and parapineal organs of adult Lampetra fluviatilis 

 A. larva, B. adult. Sagittal section. 



, inner and outer sensory cells; p. process; pin. pineal; p. pin. parapineal. 

 (After Tretjakoff.) 



Moreover, these movements can be elicited even after the pineal 

 organs have been removed! In the larval lamprey the paired eyes are 

 deeply buried below pigmented skin, so the movement is not likely 

 to be due to them; indeed it continues when they too have been taken 

 out! Evidently there must be still other receptors, able to respond to 

 changes of light intensity in the wall of the diencephalon. This recalls 

 the fact that photoreceptors are found within the substance of the 

 nervous system of amphioxus. This power of response to changes of 

 illumination has been retained in the vertebrates, and persists in some 



