504 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 an endostylar groove is constricted 

 off to form the thyroid (cf. p. 481). 



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. 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 movements 

 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 rudiments are 

 associated. 



The vascular system is essentially the same as in the 

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

 discs. 



The segmental or pronephric ducts persist as ureters, 

 and are connected with lateral mesonephric tubules forming 

 a kidney more complicated than that of the hag. The 

 pronephros, which is functional in the larva, entirely dis- 

 appears. 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. 



