PETROMYZON 



29 



pharynx. The floor of this downgrowth is folded and con- 

 tains four rows of glandular cells. It is obvious that this 

 structure is practically identical with that of the endostyle of 

 Amphioxus. Behind the pharynx, the gut leads straight back 

 through an intestine to the anus. 



During the metamorphosis from the Ammocoete larva to 

 the adult, certain important changes take place. The buccal 

 cavity develops into a sucker with horny teeth, and the rasping 

 tongue is formed in the floor of the pharynx. The endostyle 

 closes up and its glandular and ciliated cells disappear but 



Fig. 16. — Petromyzon 



transverse section through the endostyle of an 

 Ammocoete larva. 



gc, the four tracts of glandular cells ; fp, floor of the pharynx ; va, branch of 

 the ventral aorta. (Compare with Amphioxus.) 



its duct gives rise to the thyroid gland. From its habit of 

 adhering closely to its food with its sucking mouth, water 

 cannot easily pass through the animal's mouth to its gills. The 

 latter are modified into pouches which take water in through 

 their external apertures and then expel it again. Inside, the 

 pharynx becomes divided into two parts, one above the 

 other. The upper portion becomes the definitive passage 

 from the mouth to the intestine ; the lower becomes the 

 branchial tube. The latter is blind behind, receives the 

 inner openings of the seven pairs of gill-pouches, and opens 



