BREATHING MECHANISM OF THE LAMPREYS. 21 



tion of the nasal sac there continues ventro-caudad the nasal 

 ccecum which curves around the anterior ends of the brain and 

 notochord (Fig. i,/). This nasal ccecum lies directly under the 

 notochord and ends blindly on a line with the center of the sec- 

 ond sfill sac. The ventral wall of the tube follows the ethmoid 



o , 



cartilage to its posterior end and is there attached to the phar- 

 yngeus muscle as was before noted. It continues caudad from 

 this point. At its caudal end it is parallel to the long axis of 

 the body. The posterior part of the tube which is immediataly 

 above the oesophagus, overlies both its dorsal and lateral walls 

 and rests on the first and second gill sacs for I or 2 mm. on each 

 side (Fig. i). 



The result of the position of the tube directly between the an- 

 terior end of the notochord and the strong muscles of the gill 

 pouch ventral to it is that it is pressed up against the notochord 

 at every contraction of the underlying muscles. This action of 

 the muscles causes the water in the tube to be forced out 

 of the nostril with considerable force. Upon the relaxation of 

 the muscles to which the tube is attached the sac refills (Fig. 

 i). Thus, although the nasal tube has no direct communication 

 with the respiratory apparatus, yet the effect is practically as 

 though it had. At every expiration from the gills there is a cor- 

 responding expiration from the nostril and with every inspiration 

 water passes into the nostril. The relationship existing between 

 the expired and inspired streams of the nostril and gills has long 

 been known, but hitherto the causes underlying this relationship 

 have not been understood. 



( To be continued. ) 



