GEOWTH. 



175 



losing its teeth ; the spines of the head are shortened, and the 

 fins assume nearly the shape which they retain in mature indi- 

 viduals. Young Sword-fishes without ventral fins {Xiphias) 

 undergo similar changes ; and, besides, their skin is covered 

 with small rough excrescences longitudinally arranged, which 

 continue to be visible after the young fish has assumed the 

 form of the mature in other respects (Fig. 92). 



Fig. 92. — Xiphias gladius, young, about 8 inches long. 



The Plectognaths show no less extraordinary changes : 

 an extraordinary form taken in the South Atlantic, and 

 named Ostracion hoops, is con- 

 sidered by Liitken to be the 

 young of a Sunfish {Orthago- 

 riscus). In very young more 

 advanced Sun-fishes (18 to 32 

 millimetres) the vertical dia- 

 meter of the body exceeds, 

 or is not much less than, the Kg. 

 longitudinal; and small coni- 

 cal spines are scattered over its various parts. The caudal 



-" Ostracion 'boops " (much 

 magnified). 



Fig. 94. — Young of Orthagoriscus, 18 and 32 mill. long. (Natural size.) 



fin is developed long after the other vertical fins. 



