lEREGULAEITY OF GEOWTH. l79 



Sprat, Pilchard) are examples of this regular kind of growth.^ 

 But in the majority of fishes the rate of growth is extremely- 

 irregular, and it is hardly possible to know when growth 

 is actually and definitely arrested. All seems to depend on 

 the amount of food and the more or less favourable circum- 

 stances under which the individual grows up. Fishes which 

 rapidly grow to a definite size are short-Kved, whilst those 

 which steadily and slowly increase in size attain to a great 

 age, Teleosteans as well as Chondropterygians. Carp and Pike 

 have been ascertained to live beyond a hundred years. 



It is evident that such diversity and irregularity of 

 growth in the same species is accompanied by considerable 

 differences in the appearance and general development of 

 the fish. E"o instance is more remarkable than that of the 

 so-called Leptoce^phali, which for a long time have been 

 regarded either as a distinct group of Pishes, or as the larval 

 stages of various genera of fishes. 





^ ^___ . „.^^^. "^ 



IMmmm 



Fig. 95. — Leptocephalus. 



The Zeptocephali proper are small, narrow, elongate, more 

 or less band-shaped fishes, pellucid in a fresh state, but assum- 



1 This applies to individuals only growing up under normal conditions. 

 Dr. H. A. Meyer has made observations on young Herrings. Individuals 

 living in the sea had attained at the end of the third month a length of 

 45 to 50 millimetres, whilst those reared from artificially-impregnated ova 

 were only from 30 to 35 millimetres long. When the latter had been sup- 

 plied with more abundant food, they grew proportionally more rapidly in 

 the following months, so that at the end of the fifth month they had reached 

 the same length as their brethren in the sea, viz. a length of 65 to 70 milli. 

 metres. 



