EXTERNAL CHARACTERS. 221 



mouth, with which they are connected by wide 

 grooves covered by flaps of skin. 



b. The spiracles are a pair of small apertures on the 



sides of the head, just behind the openings of the 

 eyes. They are really a pair of modified gill- 

 clefts. Through the spiracles sea-water passes 

 into the pharynx. 



Pass a seeker down one of the spiracles into the mouth. 



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 and spiracles over 

 the gills for the purpose of respiration. 



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



d. The cloacal pouches are a pair of pocket-like de- 



pressions at the sides of the cloacal aperture 

 and between the pelvic fins. 



e. The cloacal papillae are attached in front of the 



cloacal pouch, with their apices directed back- 

 wards. Each is traversed by a canal (peritoneal 

 canal) which opens anteriorly into the body- 

 cavity and posteriorly by the abdominal pores 

 into the cloacal pouches. The pores are minute, 

 and are only open in mature dog-fish. 



3. Scattered apertures. 



The apertures of the sensory and ampullary canals 

 are rows of minute openings arranged symmetri- 

 cally on the surface of the head. The openings 

 of the ampullary system are especially abundant 

 on the snout. They lead into tubes of con- 

 siderable length, which lie beneath the skin and 

 are filled with a transparent gelatinous substance. 

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

 stance, and so render the openings of the tubes more obvious. 



