PETROMYZON. 



503 



of fishes, support the dorsal and caudal fins, and other 

 skeletal parts occur about the "tongue." The caudal end 

 of the notochord is quite straight. 



Nervous system. The brain has the usual parts, but is 

 small and simple ; the roof of the fore-brain is composed 

 of non-nervous epithelium ; there is a distinct pineal 

 body, with hints of an eye ; the oral part of the hypo- 

 physis is developed from in front of the mouth, and 

 becomes closely connected with the involution of epiblast 

 which forms the nostril. A unique peculiarity in the brain 

 is that the middle part of the roof of the iter is simply 

 epithelial. The spinal cord is flattened ; the anterior and 



FIG. 241. Longitudinal vertical section of anterior end 

 of larval lamprey. After Balfour. 



in., Mouth ; t/t., thyroid ; g.p. , one of the gill-pouches ; v.ao., ven- 

 tral aorta; //., heart; N., notochord ; S.C., spinal cord; ., 

 auditory vesicle; cb., cerebellum; p.f>., pineal body; c.li.. 

 cerebral hemispheres ; <?/_/., olfactory involution. 



posterior roots of the spinal nerves alternate and do not 

 unite ; there is no sympathetic system. 



Though the larva sometimes receives the name of "nine- 

 eyes " -which expresses a popular estimate of the branchial 

 apertures it is blind, for the eyes are rudimentary and 

 hidden. In the adult they rise to the surface, and are 

 fairly well developed. The optic nerves do not cross until 

 they enter the brain. The ear has only two semicircular 

 canals instead of the usual three. The single nasal sac 

 does not open posteriorly into the mouth as it does in 

 Myxine ; though prolonged backwards it ends blindly. Its 

 external opening is at first ventral, but is shunted dorsally 

 and posteriorly. 



