ALIMENTAR Y S YSTEM. 



475 



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 traces 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. The spinal cord is flattened ; 

 the anterior and posterior roots of the spinal nerves 

 alternate and do not unite; there is no sympathetic 

 system. 



fp m 



Fig. 201. — Longitudinal vertical section of anterior end 

 of larval lamprey. — After Balfour. 



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

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

 auditory vesicle; ct>., cerebellum; #.!>., pineal body; c.h. t 

 cerebral hemispheres ; olf. t olfactory involution. 



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, as 

 they usually do. 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. 



