232 



Part III. — Sixteenth Annual Report 



These were considered to be dabs ; the incompletely metamorphosed 

 because they fell within the upper of the two groups already dealt with, 

 and because they led, by gradual steps, to dabs, with all the adult 

 characteristics. 



An important point which arose at this stage was the determination of 

 the length at which the lateral line appeared. Mr. Holt states that there 

 were traces of a lateral line present in a dab of 15 •25mm., and in his 

 drawing (fig. 115) represents it as extending the ^vliole way along the 

 body. The lateral line is not developed all at once, and there are two, if 

 not three, successive stages in its appearance. It first appears on the 

 tail region, from the abdomen to the caudal fin- rays, and about the same 

 time on the head. The intermediate portion, the bend over the 

 abdomen, is not present till much later. This is the case in the flounder, 

 plaice, and dab. There are markings on the skin, lines clearer than the 

 surrounding tissue, which might be mistaken for the lateral line, but 

 closer inspection shows they are the tracings of the superficial nerves, with 

 little resemblance to the trace of the lateral line, which looks like a string of 

 beads. Sixteen specimens of the dab between 15mm. and 26mm. were exa- 

 mined. These led, by gradual steps, from the incompletely metamorphosed 

 stages to those with lateral line complete, and in their elongated oval form, 

 large eyes with large pupil, as well as their general pigmentation, showed 

 the gradual assumption of characteristics of the dab. In all these 

 specimens the tail and head portions of the lateral line were present, but 

 only in the largest, 26mm., were there signs of the markings growing out 

 from the headfportion and tail portion to meet in the curve. 



In one specimen at 24mm., and those from 26*5 onwards, the lateral 

 line was complete ; 25mm., therefore, may be taken as the average 

 length when the lateral line is complete in the dab, though the tail 

 portion is present at 15 to 17mm. 



Dr. Petersen* states that the lateral line is complete in the dab at 

 20mm., but, in all stages of metamorphosis, the dab, plaice, and flounder 

 of Danish waters are earlier (assume the characteristics at a less length) 

 than those of British waters. 



Similar examination of specimens of the plaice and flounder showed that 

 the average length for the appearance of the complete lateral line was 

 18mm. for the plaice, 14-15mm. for the flounder. In- the plaice the tail 

 portion of the lateral line can be seen in some at 13'5mm., and it is com- 

 plete in some between 16 and 17mm. 



The breadth of the dab when metamorphosis is complete was deter- 

 mined as nearly as possible. At 15 •25mm., metamorphosis hardly 

 complete, the breadth was 4mm.; at 17mm., metamorphosis complete, 

 breadth, 5^5mm. ; at 18mm., metamorphosis hardly complete, breadth, 

 5"5mm. ; at 18mm., metamorphosis complete, breadth, 6*5mm. The 

 breadth at complete metamorphosis is therefore from 5 to 6"5mm. 



At these stages, when the adult characteristics are gradually being 

 assumed by the plaice and dab. Dr. Petersen gives the size of the pupil of 

 the eye as a guide, the pupil in the dab being twice the size of that in 

 the plaice. Measurements were made of the pupil at different lengths, 

 with the following results: — 



At 14 mm. 

 15 „ 

 17 „ 

 21 to 25 „ 



Dab. 

 "3 ram, 



Plaice. 

 •3 mm. 



•35 „ 



* Loc. cit., p. 127. 



