CONSIDERATION OF INDIVIDUAL FISH 59 



the larval and post-larval changes reference should be made to 

 special works. Only the briefest summary is given here. When the 

 young plaice hatches out from the egg it is from 6 to 7-5 mm. in 

 total length, and at this stage the large yolk sac attached to the 

 ventral surface is a conspicuous object (Fig. 3, p. 58) and the young 

 fish is symmetrical, i.e. like a young cod or other round fish. For 

 some time after hatching the young plaice continues to live on the 

 food stored up in the yolk-sac. The larval period lasts from the 

 time of hatching until the asymmetrical form of the adult has been 

 attained, which is usually six weeks after hatching. During the 

 first week there is little change except for the gradual disappearance 

 of the yolk-sac, a process generally complete about the eighth day. 

 The larva now takes external food, and is from now on absolutely 

 dependent on it. Possibly this external feeding begins before the 

 yolk has been entirely absorbed. The food of the plaice at this 

 stage is necessarily of microscopic dimensions, consisting of diatoms 

 and larval moUusca. 



The young fish grows very slowly, and at the age of twenty-one 

 days from the time of hatching is only about | in. in length. After 

 this the length increases relatively slower than the height. The 

 young stages of various crustacea now form its principal food. 

 The tail begins to be bent up at the tip, the fin-rays appearing on 

 the lower or ventral surface. Up to thirty days after hatching the 

 young plaice is symmetrical. 



After the thirtieth day the left eye begins to move upwards and 

 forwards, after forty days it appears on the upper margin of the 

 head just in front of the right eye. On the forty-fifth day the left 

 eye has attained its definite position above and in front of the right 

 eye. During the period in which the eyes are rotating the young 

 plaice gradually acquires a new position in swimming. At the 

 completion of metamorphosis the whole symmetry of the head has 

 been profoundly disturbed, though that of the body remains as 

 before, with the exception of the opening of the ureter. The fish 

 finally swims and rests on what is really its left side. The pigmen- 

 tation now gradually disappears from this side. At this stage of 

 their life-history the young plaice feed on small crustacea known 

 as Copepoda, but larval molluscs and larval crustacea are also 

 eaten. After metamorphosis is complete, the food changes and they 

 feed on various worms (Annelids), and small bottom-living crustacea 

 such as various Amphipods, Mysis and small shrimps. Later the 

 adult food is sought for, and this consists mostly of moUusca of the 

 cockle and mussel family. Detailed reports on the food of plaice^ 



* See Report on the Food of the Plaice, by R. A. Todd. Board of Agriculture and 

 Fisheries. Fishery Investigations. Series II. Sea Fisheries, Vol. Ill, No. 3. 

 London, 1915. 



