298 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



perature. It is, of course, possible that some internal 

 change is produced when a fish moves from water of 

 relatively low to relatively high salinity, or vice versa : 

 the nature of the processes controlling diffusion through 

 the epithelia of the gills, or other surfaces, must undergo 

 change in such an occurrence. It is also possible that 

 there is an optimal salinity for the fish as well as an 

 optimal temperature and an optimal pressure of water, 

 but the investigations so far made do not suggest what 

 these optimal salinities are. The question of the salinity 

 is one of very great difficulty, and its investigation would 

 involve salinity determinations of the water at very 

 numerous stations, and very often throughout the year, 

 since the changes in-shore are probably relatively com- 

 plex. No attempt is, therefore, made to investigate this 

 possible relationship. 



It is almost certain that there are optimal tempera- 

 ture conditions for plaice of different ages, but it is 

 impossible, in the meantime, to say what these are. They 

 are susceptible of investigation, and might be determined 

 with a fair degree of accuracy. It was with this object 

 that samples of plaice were obtained, and the length- 

 weight coefficient k evaluated for different fishing grounds 

 and seasons. The method is certainly quite a sound one, 

 for the value of k, that is the " condition " of the fish as 

 regards nutrition, must be an index of its rate of meta- 

 bolism. No data of this kind are, however, yet available, 

 since it has been found impossible, after four years of 

 investigation, to obtain the complete series of plaice 

 samples, with the temperature records of the fishing 

 grounds when they were caught, which would allow the 

 estimates to be made. 



The hypothesis may, however, be made that there 

 are such optimal conditions of temperature, and that 



