$24 CYCLOSTOMATA. 
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 ztev is simply 
epithelial. The spinal cord is flattened; the dorsal and 
ventral 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. 
Alimentary system.—The oral funnel, at the base of 
which the mouth lies, has numerous horny teeth. It is 
applied to the lamprey’s victim, and adheres like a vacuum 
sucker ; the toothed “tongue” works like a piston; both 
flesh and blood are thus obtained. From the floor of the 
pharynx an endostylar groove is constricted off to form 
the thyroid. 
From the gullet of the young larva seven gill-pouches 
open directly to the exterior; in the adult this larval gullet 
becomes wholly a respiratory tube. It is closed pos- 
teriorly, and opens anteriorly into the gullet of the adult, 
which is a new structure. At the junction of the respira- 
tory tube with the gullet of the adult lie two flaps or 
vela. 
The rest of the gut is straight and simple, with a single- 
lobed liver, but with only a hint of a pancreas. The gall- 
bladder and bile-duct disappear-in the adult, and the whole 
intestine is partially atrophied. There is a slight spiral 
fold in the intestine. 
