x CYCLOSTOMATA 397 



Lamprey this arrangement has undergone important modification. In 

 Myxine the short tube which normally connects each sac with the exterior 

 has become considerably elongated, and the successive tubes have had 

 their openings shifted backwards along the surface of the body so as 

 to become coincident, at a point on the ventral surface of the body 

 considerably behind the position of the last sac. Consequently a Myxine 

 shows not a row of separate gill-openings along each side of its pharyngeal 

 region but merely a single pair of such openings approximated together 

 and situated well behind the region of the pharynx. 



In the adult Lamprey the pharynx has become divided longitudinally 



FIG. 174. 



Section through tooth of Lamprey (after Warren), d, Dennis ; m.p, dermal papilla ; s, functional 

 tooth ; s 1 , new tooth forming to replace s. 



into a smaller dorsally placed tube continuous with the oesophagus and 

 serving for the passage of food, and a wider' ventral respiratory tube 

 (Fig. 175, r.t), carrying the internal openings of the gill-sacs (v.c) and 

 ending blindly at its hinder end. The whole arrangement is very much 

 the same as would come about in an Amphioxus if the dorsal ciliated 

 groove which serves for the backward transmission of the food were to 

 become constricted off from the main portion of the pharynx lying 

 below it. 



There is another point of great interest about the pharyngeal region 

 of the Lamprey. During the Ammocoetes-stage an endostyle is present 

 like that of Amphioxus and serving the same function. When, however, 

 the metamorphosis into the adult condition takes place the endostyle 

 becomes constricted off from the pharnyx, so as to form a closed sac 



