390 THE FLOUNDER FAMILY. 



rolling over or advancing in a screw-like manner. While in 

 lateral view the yolk-sac is somewhat ovoid, it is quite circular 

 when seen either from the front or rear. This larva is one of the 

 most active and restless of the group seldom remaining qui- 

 escent more than a few seconds. No pigment other than the 

 superficial chromatophores exists in the eyes. The oil-globules 

 form two main groups, one series running from the heart 

 obliquely backward to the pectoral fin, the other at the 

 posterior part of the yolk, and extending ventrally at its 

 posterior border. One or two isolated groups also occasionally 

 occur along the ventral border. All retain their periblastic 

 position. 



The vesicular condition of the yolk is not readily seen after 

 hatching, though it can always be made out by careful manipu- 

 lation of the light, or in favourable positions. The vesicles 

 appear to be flattened out at the margin of the yolk. In some, 

 peculiar vesicles are observed in the yolk-sac in the neighbour- 

 hood of the posterior oil-globules. 



On the second day the yolk has considerably diminished, 

 and its posterior border moving forward carries the oil-globules 

 with it, leaving a larger space between it and the vent. Minute 

 pigment-specks now appear in the eyes. The segments of the 

 yolk are still indicated. 



On the fourth day the yolk has still further shrunk. The 

 cavity of the mouth is formed, though no external aperture yet 

 exists, and the gut terminates blindly a little within the 

 margin of the fin. The remarkable size of the optic lobes 

 of the brain gives the larval sole a " hooded " aspect, and its 

 agility is characteristic. 



Three days later, that is on the 7th day, the activity of the 

 larval fish is even more marked, and it seems in a state of 

 perpetual movement, the only interval being for a second or 

 two after a long course through the vessel. This almost 

 ceaseless movement is probably connected with respiration, 

 the now widely open mouth being driven against the water 

 which rushes into it. Of course were the larval fish in currents 

 sufficiently strong, its habit would be different, as it would 

 simply keep its head against the stream, as in the young plaice 



