io8 



VERTEBRATES WITHOUT JAWS 



IV. II- 



it represents the cavity of the hypophysis and prefer the name 

 nasopalatine canal. 



Inside the single nostril, guarded by a valve, are openings into the 

 nasal sacs, which are cavities with folded walls. Some of the cells of 

 these walls are the olfactory receptors and give off the axons that make 

 up the olfactory nerves, entering the olfactory bulbs on the anterior 

 end of the hemisphere (Fig. 65). Behind the nasal sacs lie numerous 

 glandular follicles opening into the sac in the larva, but completely 

 closed in the adult (Fig. 65). They may be comparable to Jacobson's 

 organ (p. 405). 



Fig. 71. Section of lateral line organ of tail of adult Lampetra. 



p. pigmented cells around the pit; s.c. receptor cells 

 (not showing long hairs). (After Young.) 



The naso-hypophysial tube proceeds back behind the pituitary to 

 a closed sac lying between the first pair of gill pouches (Fig. 70). 

 During the movements of respiration this sac is squeezed and water 

 is expelled with some force through the nostril. When the gills relax 

 water flows in at the nostril, and in this way the olfactory organ is 

 provided with samples. If the naso-hypophysial opening is closed with 

 a plug of plasticine the lamprey no longer reacts to solutions, for 

 instance of alcohol, to which it normally responds by freeing its 

 sucker and swimming away. 



12. Lateral line organs of lampreys 



The lateral line receptors, peculiar to fish-like vertebrates, are little 

 patches of sensory cells found along certain lines on the head and 

 trunk. They are all innervated by cranial nerves, those on the body 

 and tail being served by a special backward branch of the vagus nerve. 

 The receptor cells carry long hairs and are thus able to detect either 

 movement of the water relative to the fish or of the fish itself. Objects 

 moving nearby set up disturbances that may also be detected (p. 218). 



