CHAPTER VI 



LIFE HISTORIES 



Life-histories of Teleostomi. Eggs of Clupeidae. Metamorphoses of Angler 

 and Pleuronectidas. Growth and Maturity. Diseases and Parasites. Life- 

 history of Eels. 



THE Elasmobranchs, or fishes of the shark type, and 

 Chimaeroids when hatched or born have already acquired 

 the characters of the adult, and this is approximately 

 true of the new-born young of the viviparous species among 

 bony fishes. But in those of the Teleostomi which are hatched 

 from eggs, the young are more or less different from the adults, 

 and have to undergo various changes of structure and habits 

 before they reach the adult condition. The changes may be 

 greater or less in degree and may be more gradual or more 

 abrupt in development in different types. The young stage of 

 an animal when thus distinctly different from the adult is 

 called a larva, and the development of the larva into the adult 

 form is its metamorphosis. The most familiar examples of 

 metamorphosis are those of the tadpole into the frog and of the 

 caterpillar into the moth or butterfly, and in some bony fishes 

 the change is as remarkable in its own way as it is in these two 

 cases. 



In all ordinary cases the metamorphosis is gradual and 

 consists chiefly in the development of the fins and fin-rays and 

 of the scales and internal skeleton. 



In the hatched larva there is usually a continuous mem- 

 brane along the median line of the back, round the tail, and 

 along the ventral edge as far as the anus or even some distance 

 in front of the latter ; the paired fins are also simple membranes. 

 The larva of the lung-fishes (Dipnoi) is somewhat like a tad- 

 pole ; in the Australian Ceratodus there are no external gills, 

 but in the African Protopterus and the South American Lepido- 

 siren there are four pairs of feathery outgrowths quite similar to 



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