GROWTH. 171 



sistent than in Acanthopterygians. A metamorpliosis relating 

 to the respiratory organs, as ia Batrachians, is indicated in 

 the class of Fishes by the external gills with which foetal 

 Plagiostomes (Fig. 58, p. 136) and the young of some Ganoids, 

 viz. the Protopterus and Polypterus, are provided. 



One of the most extraordinary changes by which, during 

 growth, the form and position of several important organs 

 are affected, occurs in Flat-fishes (Pleuronedidce) ; their young 

 are symmetrically formed, with a symmetrical mouth, and 

 with one eye on each side, and, therefore, keep their body in a 

 vertical position when swimming. As they grow they live 

 more on the bottom, and their body, during rest, assumes a 

 horizontal position ; in consequence, the eye of the lower side 

 moves towards the upper, which alone is coloured; and in 

 many genera the mouth is twisted in the opposite direction, 

 so that the bones, muscles, and teeth are much more developed 

 on the blind side than on the coloured. In a great number of 

 other Teleostei certain bones of the head show a very different 

 form in the young state. Ossification proceeds in those 

 bones in the direction of lines or radii which project in the 

 form of spines or processes; as the interspaces between 



Fig. 84.— Armature of prseoperculum of young Caranx ferdau. (Magnified.) 

 I. Of an individual, li inch long. II. Of an individual, 2 inches long. 



these processes are filled with bone, the processes disap- 

 pear entirely, or at least project much less in the older than 

 in the younger individuals (Fig. 84). The young of some fishes 

 may be armed with a long powerful praeopercular or scapular 

 spine, or show a serrature of which nothing remains in the 



