232 MARKETABLE BRITISH MARIXK FISHES 



at Plymouth and elsewhere. The eggs hatched seven days after 

 fertilisation at Plymouth at a temperature of about 53 (12 C). 



The larva when hatched is 2'5 to 3 mm. long (12 hundredths 

 of an inch). The mouth is not open ; the pigment is yellow and 

 black, and is absent from the yolk-sac and fin-membrane. A 

 larva two days old (Fig. 59) was found to be y$6 mm. long (not 

 quite i inch). The yolk was reduced to half its original size, 

 and the mouth was nearly but not quite formed. At this stage 

 the pigment had extended on to the marginal fin-membrane, and 

 formed a single broad band across the tail, which is character- 

 istic of the larval flounder. It also formed a less distinct band 

 at the level of the vent. 



At six days of age (Fig. 60) the larva was 3-94 mm. long (not 

 quite % inch or 16 hundredths), the yolk was all used up, the 

 mouth was formed, and was at the end of the snout. The band of 

 pigment across the tail still remained, the pectoral fin was a large 

 rounded membrane. No indication of fin-rays or skeleton was 

 yet visible. 



These stages have been studied from larvae hatched arti- 

 ficially and kept under observation. The later stages are known 

 from specimens captured on the shore. At many places young 

 flounders in various stages of their transformation, and just be- 

 yond it, occur in great abundance in April and May in the 

 shallow pools and channels left by the ebb of the tide. They 

 have been observed and collected at St. Andrew's in Scotland, 

 and at Mevagissey in Cornwall. The species can be distinguished 

 without difficulty by the number of the fin-rays, for these are 

 formed all at once and not gradually : no other flat fish in 

 British waters has so few as forty-five rays in the ventral fin. 

 Fig. 6 1 shows the structure of one of these young flounders at a 

 stage in which the left eye has not quite reached the edge of the 

 head in its rotation, and Fig. 62 shows the later stage in which 

 the eye is on the edge of the head. 



The young flounders in the third period of life, from complete 

 development to maturity, are found in rivers, harbours, and estu- 

 aries, which they never leave until they go to sea for the first 

 time, to spawn. Details of the experiments on the rate of growth 

 of flounders have been given in a previous part of this book. 

 The young fish in the estuaries have not yet been very carefully 

 studied. At the mouth of the Humber, below Grimsby, during 



