5*4 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 iter 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. 



