24 Distribution of the Smaller Crustacea. [Sess. 



influence of the seasons on the distribution of the smaller 

 crustacean species : this is sometimes more easily observed in 

 regard to fresh- water forms than amongst those that live in 

 the open sea. Second, reference will be made to the occur- 

 rence of certain species ofif the Scottish coasts which seem to 

 owe their presence there to the influence of oceanic currents. 

 And, third, a number of species — chiefly parasitic — will be 

 referred to, the distribution of which appears, so far, to be 

 inexplicable. 



First, The influence of the seasons on the distribution of 

 the smaller Crustacea. Though the changes of the seasons 

 have, no doubt, a certain influence on the distribution of the 

 smaller crustacean species, it is not always easy to ascertain, 

 even approximately, how far this influence may be exerted, 

 or to what extent the ordinary seasonal influences may be 

 neutralised by changes that are accidental and temporary. 

 Only by continuous observations extending over a number 

 of years can any satisfactory knowledge concerning the influ- 

 ence of the seasons on distribution be acquired. But though 

 such a study may be difficult, and the results sometimes dis- 

 appointing, it is nevertheless full of interest ; and one of the 

 reasons which make it unusually interesting is due to the 

 unlooked-for incidents which are to be met with occasionally. 

 I have stated that the ordinary seasonal influences may be 

 interfered with by accidental and temporary changes : the 

 following example will show how this may occur. The 

 various species of Daphnia are, under ordinary circumstances, 

 not greatly affected by seasonal changes. I have found them 

 in Daddingston Loch, Loch Leven (Kinross), Forfar Loch, 

 and other lochs which I have visited at various seasons, to 

 be nearly, though not quite, as numerous in winter as in 

 summer. (See Part IIL of the 16th Annual Eeport of the 

 Fishery Board for Scotland, p. 132 et seq.) But though the 

 distribution of Daphnia ■ in these lochs did not appear to be 

 greatly affected by seasonal changes, it was otherwise with 

 those observed in an artificial pond near Comely Bank, Edin- 

 burgh. I visited this pond, with a friend, on July 3, 1898, 

 and found Daphnia pulex abundant; on the 25th of the 

 following month, or a little more than seven weeks after 



