EXTERNAL APEETUEES 199 



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 obliquely 

 backwards, and during life 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 apertures 



opening into pocket-like depressions, at the sides 

 of the cloacal aperture, and between the pelvic 

 fins. They communicate with the abdominal 

 cavity, and are best seen from within (p. 217) ; 

 they are often closed in young specimens, espe- 

 cially in females. 



8. Scattered apertures. 



a. The apertures of the mucous canals are rows of 

 minute openings arranged symmetrically on the 

 surface of the head, and especially abundant on 

 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 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 ate special de- 

 velopments 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 back- 

 wards, 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. 



