334 A HISTORY OF FISHES 



but soon loses its caudal fin and acquires a regular armour of 

 strong spines projecting in every direction all over the body, 

 which scr\c to protect it during a period of helplessness 

 (Fig. i20f). Five of these spines afterwards grow out into 

 long "horns," one of which projects from the middle of the 

 back, one from the snout, one from the chest, and one from 

 each side of the body (Fig. i20e). A httle later the fish under- 

 goes a remarkable change in shape, the body actually becoming 

 deeper than long; the spines shorten, and a new tail-fin develops 

 which connects the abbreviated dorsal and anal. The fish is 

 now about half an inch in length, and from this stage onwards 

 it gradually assumes the form of the adult. 



The young of the Deal-fish ( Trachypterus) is remarkable for 

 the extraordinary development of the fin-rays, those of the 

 front part of the dorsal, of the pelvics, and of the lower lobe of 

 the caudal being produced into very long filaments, which 

 may be many times longer than the body and are ornamented 

 with lappet-like membranous processes (Fig. 121 bI). As the 

 fish grows these filaments get progresively shorter and the 

 lower lobe of the caudal fin disappears (bII). The Deal-fish is 

 an oceanic species, and it seems probable that the young live 

 at some considerable depth where the water would be fairly 

 calm, for the currents prevailing at or near the surface of the 

 sea would soon damage such delicate structures. 



The Sword-fishes [Xiphiidae) and their aUies, the Sail-fishes 

 and Spear-fishes {Istiophoridae) , are distinguished by having the 

 snout prolonged to form a long flat or rounded spear or sword, 

 and the changes undergone by these fishes during development 

 are very striking. The young of the Sail-fish {Istiophorus) have 

 been beautifully illustrated by Dr. Giinther, whose figures are 

 reproduced here. In the first stage (Fig. 121 aI), an individual 

 nine miUimetres in length, both jaws are equally produced and 

 armed with pointed teeth ; the edge of the head above the eye 

 is provided with a series of short bristles ; and from the back of 

 the head project, above and below, long pointed spines. The 

 dorsal fin is a long low fringe, the pectoral is large and truncated, 

 and the pelvics are represented by a pair of short buds. In the 

 next stage (aII), a fish fourteen millimetres long, the dorsal has 

 increased enormously in size, the pelvics have grown out into 

 long filaments, and the pectorals have changed their shape. 

 The spines on the head are still prominent, but the bristles 

 above the eyes have disappeared, and the upper jaw has grown 

 a litde longer than the lower. At the third stage (aIII), when 



