476 CYCLOSTOMATA. 



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 a groove is constricted off 

 (cf. p. 453). 



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 re- 

 spiratory 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. 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 movements 

 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 rudiments are 

 associated. 



The vascular system is essentially the same as in the 

 hag. The red blood cells are biconcave, circular, nucleated 

 discs. 



There are two elongated kidneys (mesonephros), each 

 with a wide ureter. The ureters unite terminally in a 

 urogenital sinus (not present in the hag), into which there 

 open two genital pores from the body cavity. The sinus 

 opens, like the anus, into an integumentary cloacal 

 chamber. 



Reproductive system. — The sexes are separate. The 

 reproductive organ is elongated, unpaired, and moored 

 by a median dorsal mesentery. There are no genital 

 ducts. The ova and spermatozoa are liberated into 



