EXTERNAL APEBTUBES 197 



c. The gill-clefts are a series of five vertical slits on 



each side of the neck, in front of and slightly 

 above the pectoral fins. They open obUquely 

 backwards, and during Hfe give exit to the water 

 passed from the mouth over the gills for the 

 purpose of respiration. 



Pass a seeker through the gill-clefts into the mouth. 



d. The abdominal pores are a pair of small pocket-like 



depressions situated between the pelvic fins, and 

 immediately behind the cloacal aperture. They 

 communicate by small openings with the abdo- 

 minal cavity. 



8. Scattered apertures, 



a. The apertures of the mucous canals are rows of 

 minute openings arranged symmetrically along 

 the surface of the head, and most abundant in 

 the snout. They lead into tubes of considerable 

 length — the mucous canals— which lie beneath 

 the skin, and are filled with a transparent gela- 

 tinous substance. 



Squeeze the head so as to press out the gelatinous sub- 

 stance, and so render the position of the openings of the tubes 

 more obvious. 



D. The Scales and Teeth. 



1. The scales. The whole surface of the body is covered 

 with small placoid scales, which are special develop- 

 ments of the skin. 



a. General arrangement. The scales are larger on the 

 dorsal than on the ventral surface, and differ 

 somewhat in shape in different parts of the body. 

 Their projecting points are directed backwards, 

 so that the finger can be readily passed along the 

 body from the head to the tail, but meets with 

 considerable resistance in the reverse direction. 



