582 PHYLUM CHORDATA : CLASS CYCLOSTOMATA 



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 open 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. 



Respiratory, vascular, and excretory systems. — 

 Seven gill-pouches with plaited walls open directly to the 

 exterior on each side, and communicate indirectly with the 

 gullet. Water enters the pouches partly via the mouth, 

 partly by the external apertures (spiracula), and the move- 

 ments of the branchial basket and of the tongue-piston aid 

 greatly in the process. In the larva there is an eighth most 

 anterior pouch which does not open to the surface. It 

 corresponds to the spiracle of Elasmobranchs. With each 

 of the seven open pouches in the larva four thymus rudi- 

 ments are associated. 



The vascular system is essentially the same as in the 



