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 (2) 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 
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 ¥ oo 
view of Spanish rals may acquire additional uses: they may 
ee serve 4s 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 
plane they may, like the unpaired fins, funec- 
