CLASS AGNATHA 



363 



Teeth 



Oral funnel 

 Tongue 



Mouth cavity — 

 Protractor musci 



of tongue 



Velum 



Tongue cartilag 



Pharynx 



Inferior jugular 

 vein 



Ventral 

 Esophagus 



Cross section through 

 gill pouches (arrows 

 indicate v/oter current) 



Retractor muscle 



of tongue 



External gill slit 



Gill filaments 



Internal gill slit 



Nerve cord 

 ?;^Notochord 

 ■'^Jugular vein 



Dorsal aorta 



Ventral aorta 



Ventricle 



Atrium 



Sinus venosus 



Hepatic vein 

 Liver 



Testis 

 Intestine 

 Dorsal aorta 

 Liver 

 Coelom 



Mouth 



Nasal 

 opening 



Olfactory sac 



Brain 

 Pituitary sac 



Esophagus 



Dorsal aorta 

 Nerve cord 



Notochord 



Jugular vein 



Posterior 

 cardinal vein 



Intestine 

 Testis 



Muscle 



Posterior 



cardinal vein 



Nerve cord 



Notochord 



Kidney 



Figure 242. Petromyzon, an adult lamprey. Shown in cross section and with left side of body 

 removed to demonstrate the structure. 



to the diencephalon; ventral to the latter is 

 a broad infundibulum, and above it a pineal 

 structure. On the midbrain is a pair of large 

 optic lobes. In the hindbrain, the rudimen- 

 tary cerebellum is a small, dorsal, transverse 

 band, but the ventral medulla oblongata 

 (medulla) is fairly well developed. There 

 are only 10 pairs of cranial nerves as the 

 terminal nerve is absent. The nerve cord is 



flat and lies on the floor of the neural canal. 

 There are no sympathetic ganglia, and the 

 autonomic system consists only of an in- 

 testinal plexus linked with the brain. 



Sense organs 



Organs of taste, smell, equilibrium, and 

 sight are present in the lamprey. The end 



