A HISTORY OF FISHES 



no resistance to forward motion, presenting a comparatively 

 smooth surface, which is still further improved by the presence 

 of a copious supply of slime. This mucous covering is designed 

 to reduce friction with the surrounding water to a minimum, 



Fig. 6. — " SWORD-FISHES." 



A. Sail-fish (Istiophorus americanus) ; b. Spear-fish {Makaira mitsukurii) ; c. 

 Sword-fish (Xiphias gladius). All much reduced. 



not only on account of its inherent slipperiness, but also because 

 it fills up any small irregularities in the surface of the body. 

 Finally, the smooth hollow curves of the hinder end of the 

 body, extending from the region of greatest thickness back- 

 wards to the tail, are admirably adapted to permit of the 

 passage of the water displaced during forward motion. 



