THE NOSK 



241 



or grooves in the skin of the upper surface of the head. In 

 all Fishes two of these mere blind nose-pits are found in 

 the upper surface of the head ; sometimes they are situated 

 at the back, near the eyes, sometimes near the snout, or, 

 again, near the mouth-opening (Fig. 191, 71, p. 113). They are 

 lined by mucous membrane in folds, over vrhich the end 

 branches of the olfactory nerves spread. 



In this its original condition the double nose of all 

 Amphirhina (p. 101) is entirely unconnected with the pri- 

 mitive mouth-cavity. "The connection, however, begins to 



7n 



Fig. 231.— Head of a Shark {Scyl- 

 Hum), from the ventral side : m, mouth 

 opening ; o, nose grooves, or pits ; r, 

 nasal furrow; w, nose-flap in its 

 natural position ; n', nose-flap turned 

 up. (The dots are openings of mucous 

 ducts.) (After Gegenbaur.) 



appear even in some Primitive Fishes (Selachii) ; a super- 

 ficial skin-furrow extends on each side from the nose-groove 

 down to the adjacent corner of the mouth. This furrow, 

 the nasal channel, or furrow (Fig. 231, r), is of great sig- 

 nificance. In many Sharks (e.g., Scyllium) a special process 

 of the frontal skin, the nasal flap, or " inner nasal process," 

 overlaps the nasal furrow (n, n'). Opposite to this the outer 

 edge of the furrow rises and forms the "outer nasal process." 

 In Dipncusta and Amphibia these two nasal processes meet 

 over the furrow and coalesce, thus forming a canal, the 

 " nasal canal." There is now a passage from the external * 

 nasal groove tli rough this canal directly into the mouth- 



