EXTERNAL CHARACTERISTICS 9 



which is marked by parallel ridges representing a rudi- 

 mentary gill. The mouth is on the ventral side and is 

 limited by the tooth-bearing upper and lower jaws. An- 

 teriorly from the corners of the mouth running to the 

 nostrils is a flap, the nasofrontal process, under which is 

 the oronasal groove. This groove foreshadows the develop- 

 ment of the closed passage found in the more specialized 

 vertebrates. There are five pairs of gill-slits posterior to 

 the mouth. These may be designated as the second, third, 

 fourth, fifth, and sixth gill-slits. 



4. Tail. 



The cloacaJ aperture or anus (the term anus should be 

 restricted to the opening of the digestive tract) is located 

 between the bases of the pelvic fins. Abdominal pores 

 which lead into the body cavity are located just posterior 

 to and on each side of the aperture. Their use is obscure, 

 but they may represent the remains of segmental ducts. 



5. The Skin. 



A. The Placaid Scale. 

 The skin is quite tough. It has scattered placoid scales 

 whose projecting spines are very evident. The placoid 

 scale occurs in the elasmobranch fishes and is regarded as 

 the ancestral structure from which true vertebrate teeth 

 were developed. Remove a piece of skin containing one 

 placoid scale. Clean away the skin and expose the com- 

 plete scale, which consists of a basal plate embedded in the 

 skin, and a projecting curved spine. Within the spine is a 

 pulp cavity, which may be located by probing with the 

 point of a needle at the center of the under side of the 

 basal plate. The coating of the spine is composed of 

 enamel. The basal plate and the interior of the spine are 

 composed of dentine. 



