THP] ELAHMOBRANCII FISHES 7 



Overlying the coat of drab and the pattern of dots there is, in an adnlt, an 

 armament of denticles, the so-called plaeoid scales, myriads of which go to 

 make np the protective shagreen exoskeleton (see p. 24, fig. 27 ) . This armament 

 in the adult Heptanchus is made up of a vast number of closely set scales, 

 many of which are more or less spade-shaped. In the more exposed parts of 

 the body, however, which are subject to great wear, the scales often become 

 modified and plate-like. 



Another characteristic made out in external view is the lateral line {II., 

 fig. 15). In Hepianchus, this line is an open groove extending from the end of 

 the tail along the side of the body to the pharyngeal region. This groove is con- 

 nected with certain canals of the head, w^iich, as closed tubes, are located 

 deeper in the integument. Branching from the canals are small chimney-like 

 tubes which retain connection with the surface by pores. 



In the walls of this system of grooves and canals are groups of sense organs 

 which we shall consider more fully in Chapter X. 



In addition to these lines of pores there are other aggregates of pores (for 

 example, p. 260, fig. 227a, soa.) in the region of the head, and pits which are 

 located along the dorsal and anterior parts of the trunk. Each of these pores 

 is the entrance to a tube wiiich leads to an enlargement or ampulla of Loren- 

 zini. The tubes are filled with a jelly-like mucus which, if pressure be put on 

 the skin, may be made to exude from the pores. It is from this content that the 

 pores of the ampullae of Lorenzini and those of the canal system are known 

 as mucous pores. 



