Embryology and Growth of Fishes 143 



development of the vertical fins persists for a considerable 

 time. In many young fishes, especially the Chcetodontidce and 

 their allies (butterfly-fishes), the young fish has the head armed 

 with broad plates formed by the backward extension of certain 

 membrane-bones. In other forms the bones of the head are 

 in the young provided with long spines or with serrations, which 

 vanish totally with age. Such a change is noticeable in the 

 swordfish. In this species the production of the bones of the 

 snout and upper jaw into a long bony sword, or weapon of offense, 

 takes place only with age. The young fish have jaws more 

 normally formed, and armed with ordinary teeth. In the head- 

 fish (Mola mold) large changes take place in the course of growth, 

 and the young have been 

 taken for a different type of 

 fishes. Among certain soft- 

 rayed fishes and eels the young 

 is often developed in a pecu- 

 liar way, being very soft, 

 translucent, or band-like, and 

 formed of large or loosely 

 aggregated cells. These pecu- 

 liar organisms, long known as FlG - 108. MoZa mola (Linnaeus). Very 

 i 11-1 1 early larval stage of the Headfish, called 



leptocephali, have been shown ,., 



Centaurus boops. (After Richardson.) 



to be the normal young of 



fishes when mature very different. In the ladyfish (Albuld) Dr. 

 Gilbert has shown, by a full series of specimens, that in their 

 further growth these pellucid fishes shrink in size, acquiring 

 greater compactness of body, until finally reaching about half 

 their maximum length as larvae. After this, acquiring essentially 

 the form of the adult fish, they begin a process of regular growth. 

 This leptocephalous condition is thought by Giinther to be due to 

 arrest of growth in abnormal individuals, but this is not the case 

 in Albula, and it is probably fully normal in the conger and other 

 eels. In the surf -fishes the larvae have their vertical fins greatly 

 elevated, much higher than in the adult, while the body is much 

 more closely compressed. In the deal-fish (Trachypterus) the 

 form of the body and fins changes greatly with age, the body 

 becoming more elongate and the fins lower. The differences be- 

 tween different stages of the same fish seem greater than the 



