1 66 BRITISH FISHERIES 



the case of a bony fish, the creature which hatches 

 from the egg is called a " larva/' and is one of the 

 most helpless of organisms. It has very feeble 

 powers of locomotion, and for a time it is physi- 

 cally incapable of feeding, for the mouth may not 

 be formed, and the oesophagus or gullet is usually 

 not a tube, but a solid cord of cells. This con- 

 dition persists for about a week or fortnight, and 

 the larva depends for its nutrition on the food con- 

 tained in the yolk of the ovum from which it is 

 formed. When the fish is hatched this food-yolk 

 is contained in the " yolk-sac/' a prominent 

 rounded structure projecting from the belly of the 

 larva, and which gives it a most characteristic 

 appearance. At the end of about a fortnight this 

 yolk-sac completely disappears, during which time 

 the larva has been growing actively. It is now 

 able to find and devour its own food, which con- 

 sists naturally of very minute organisms diatoms, 

 micro-crustacea such as copepods, and the very small 

 larva of various invertebrate animals. When it has 

 grown for some time the " metamorphosis " takes 

 place. It has hitherto been quite unlike the parent 

 fish ; the larva of a flat fish, for instance, like the 

 plaice or sole, is almost exactly similar to the larva of 

 a cod, is round, and has one eye on each side of its 

 head. With the metamorphosis the whole appear- 

 ance of the fish is changed. The round fish, like 

 the cod or herring, takes on the adult colour and 

 conformation, and the larva of the flat fish flattens 



