404 ee ee NB ING 
In the first group of hauls, which we may call the end of May group, 
the size of greatest frequency is 6-5 mm. and the numbers form a fairly 
regular curve about this mode, the average size of all the measurements 
being 7-15 mm. In the second group of hauls, the middle of June group, 
the numbers range themselves round two centres of frequency or modes, 
one at 6-5 mm., the other at 95mm. Although the numbers are not 
perhaps sufficiently large to be conclusive, they at least suggest that these 
9-5 min. fish belong to the same batch of larvee as the 6-5 mm. fish of the 
end of May group, whilst the smaller fish represent a new batch of young, 
derived presumably from another shoal of spawning Mackerel. If this 
be so, we should, I think, be not far wrong in concluding that a growth 
of 35mm. took place in the three weeks’ interval from the middle of the 
period May 25th to June 3rd and the middle of the period June 10th to 
June 29th. 
I give this actual instance from our own observations merely as an 
illustration of the way in which the problem of the rate of growth of 
larvee in the sea under natural conditions may be attacked. To arrive 
at perfectly certain and definite results a much larger number of speci- 
mens would be necessary, and confirmation in different years would be 
required, 
The Cod and the Mackerel, whose larval growth we have just been 
considering, are typical instances of ordinary round fishes. We will 
now look for a moment at the flat-fishes in which a distinct and rapid 
change both in structure and habit of life takes place at the end of the 
larval period. The young fish abandons its pelagic existence, during 
which it swam freely through the mass of the water snapping here and 
there at the small floating creatures upon which it feeds, and takes to 
lying on its side on the sand at the bottom, feeding on small worms, shell- 
fish and crustaceans which the sand contains. This change of habit is 
accompanied by a twisting of the whole structure of the skull, in such 
a way that both eyes come to lie near together on one side of the fish,— 
on the coloured side which is uppermost as the fish rests upon the sand. 
| A series of slides showing the metamorphosis of a flat-fish was shown. | 
This change in structure and habit gives us a fixed point in the life- 
history of the fish, and for the purpose in hand we require to know for 
what length of time the free-swimming larval stage is continued, from 
the time that the larva leaves the egg until it settles down as a little flat- 
fish adapted to life on the sand. Unfortunately the evidence available 
is not sufficiently detailed to enable us to fix this time with the degree 
of accuracy which we should desire, though an approximate estimate of 
its duration can be made for certain species. For instance, the Plaice 
in the southern part of the North Sea commences to spawn in January ; 
