26 Distribution of the Smaller Crustacea. [Sess. 



susceptibility to seasonal influence is Diaptomus laciniahis, 

 Lilljeborg. This species was found in Loch Doon, in Ayr- 

 shire, but only in September 1897 and July 1898, while 'in 

 the intervening months of December and March no trace of it 

 could be detected. The distribution of Dlapt&nius gracilis, 

 G. O. Sars, is very different. I have found this species not 

 only in most of the lochs examined, but have gathered it all 

 the year through. There are other fresln-water species which 

 exhibit a tendency to seasonal change in their distribution, 

 but the change does not appear to be so marked as in those 

 already alluded to. 



The effects of the seasons on the distribution of the marine 

 Crustacea are no doubt also considerable, but other changes, 

 accidental and temporary, may occur which may so obscure 

 and neutralise those more regular changes which the seasons 

 usually produce as to cause them partly or wholly to escape 

 our observation. Continuous stormy weather, for example, 

 gives rise to currents, which may be comparatively cold or 

 warm according to the direction from which the wind has 

 been blowing, and these currents, pushing their way along our 

 coasts, alter for a time the normal temperature of the sur- 

 rounding water, and so react on both animal and vegetable 

 life. But such currents will not only have a certain influence 

 on the local fauna, — they may also be the means of bringing 

 occasionally within our faunal limits, and even into our estu- 

 aries, rare and interesting organisms whose usual habitat is 

 beyond the British area. 



Many examples might be given which seem to indicate the 

 effect of seasonal change on the smaller marine Crustacea, but 

 the following may suffice. In some gatherings of small Crus- 

 tacea sent from the Clyde in 1901, Pod oji ZeucJMrtii {Gr. 0. 

 Sars), one of the cladoceran species, was found moderately fre- 

 quent in those collected in the spring months, but not in those 

 collected later in the year. On the other hand, Podon inter- 

 medius (Lilljeborg) and Podon polyphemoidcs (Leuckart) were 

 observed only in the later gatherings. In a paper on some of 

 the results of the investigations carried on in the Firth of 

 Forth by the Fishery Board for Scotland I have shown, for the 

 seven years from 1889 to 1895, both inclusive, that the maxi- 

 mum abundance of Calani in their young free-swimming stage. 



