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Part TTT. — Fifteenth Annual Report 



The post-larval plaice remained swimming on the edge vertically till the 

 left eye had begun to move upwards. After that time the body took a 

 more and more sloping position as the eye approached the edge, and it 

 appeared as if the right and left eye were always kept on a horizontal 

 h vel with one another. As far as I could ascertain, the duration of this 

 transformation or change from vertical to horizontal position of the body 

 did not exceed three days and a half. 



In Table!, is summed up some of the principal features during the post- 

 larval development that have been recorded in the previous pages. It must 

 be observed that all measurements are taken from what was considered an 

 average specimen at the different times, and as these were subsequently 

 preserved, I had not an opportunity of following one single larva all 

 through its development till the transformation took place. 



The average time that elapsed from the hatching till the transformation 

 is given = 41 days from my experiment, but the duration of the post-larval 

 stage varied considerably. The shortest time recorded was 37, and the 

 longest 48 days. [This is respectively 29 and 40 days after the yolk had 

 been absorbed.] 



Though brought up under somewhat artificial conditions, I find that the 

 size of my larva3 in the transformation stage corresponds with Petersen's 

 (/. c, 93, p. 126) observations made on the Danish east coast from speci- 

 mens captured in the sea. He gives the average size at the termination 

 of the post-larval stage as 10 to 11 mm. It appears that Holt (Z. c, p. 

 76-79) on the Irish coast found plaice in the transformation stage of a 

 similar size as mine, or slightly longer (10 to 13 mm.), while Ehrenbaum 

 (I. c, p. 265) has recorded post-larval forms measuring from 13 to 17 mm., 

 with the eye still looking to the left. Ehrenbaum's observations were 

 made on specimens captured near Heligoland. The smallest sized plaice 

 that he found on the bottom measured 138 mm. 



From Table I. -it will be seen that the larvae increased little in size 

 during the first 18 days after the yolk had been absorbed, though they were 

 taking abundant food, and no great changes of appearance were observable 

 during this period. It is noteworthy that just this period appeared the 

 most difficult for them to pass. But as soon as the skeleton began to 

 develop, an increase in growth, and particularly in breadth, took place, and 

 the latter was especially noticeable during the transformation. I have 

 observed that the plaice grows very quickly soon after it has become 

 stationary on the bottom ; and this seems natural, as this flat-fish spends its 

 early life on the shallow sandy beaches, where the temperature of the water 

 is often very high. 



Before closing this chapter it may be of interest to review the records of 

 food already given under the various periods into which the post-larval 

 stage has been divided. As will be seen, diatoms were eaten by some 

 larvae during the first days of this stage, while a greater number at once 

 began to feed on larval molluscs. Whether these had previously been 

 feeding on diatoms, and were eaten by the fry for the sake of the latter, I 

 cannot say, but do not think it at all probable. It was clear that the 

 living diatoms played a comparatively small part as an article of nourish- 

 ment to the larvae, much less than what has generally been anticipated.* 

 From the records it will also appear that relatively few forms of organisms 

 were eaten of the many present. 



The single individuals seemed to confine themselves to one or two forms 

 at the time, particularly in the early stages, and this was repeatedly 



* Ehrenbaum (I. c, p. 266), found remains of diatoms (Coscinodiscus sp.), 

 and afterwards remains of copepoda in some stomachs from plaice, in the later part of 

 the post-larval stage. These lame had been captured in the sea. 



