4 FISHES IN GENERAL 



rupted posteriorly, where useless elements have been dis- 

 carded ; the caudal is broadly forked, stout in its support- 

 ing rays, strong in power of propulsion. At its sides a 

 remarkable ridge has been developed, functioning as a 

 horizontal keel {R) and preventing the stroke of the cau- 

 dal from varying from the vertical plane. 

 The lateral, or paired fins, pectoral and ven- 

 tral {P and V), may rotate outward and 

 arrange themselves in the line of the fish's 

 motion, so that in a somewhat horizontal 

 plane they may, like the unpaired fins, func- 

 tion as keels. When thus erected, the 

 paired fins present a firm anterior margin 

 which serves as a cutwater. While thus 

 somewhat similar in function to the vertical 

 fins, the ventrals and especially the pecto- 



Fig.4. — Front . . . 



view of Spanish rals may acquire additional uses : they may 



mackerel. i t ^ i 1 • 1 • 



serve as delicate balancers, or may aid in 

 guiding or arresting the fish's motions. 



In further conformity to aquatic needs, the entire sur- 

 face of the fish is notably slime covered, and although 

 perfectly armoured by plates and scales, yet presents no 

 point of resistance to forward motion. An internal balance, 

 moreover, has been effected between the supporting, vis- 

 ceral and muscular parts : the firm vertebral axis acquires 

 its central position, and at its anterior end the head struct- 

 ures form a compact, wedge-like mass : the body muscles 

 which give the fish its form-contour thin away on the ven- 

 tral side, permitting in the region between the head and 

 the anal fin the space occupied by the closely compacted 

 viscera : respiratory organs occupy a restricted space on 

 either side of the gullet ; the heart and its arterial trunk 

 are implanted closely in the throat in the median ventral 



