4 Distribution of the Smaller Crustacea. [Sess. 
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 Diaptomus 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 fresh-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, Podon Leuckartw (G. O. 
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 wnter- 
medvus (Lilljeborg) and Podon polyphemoides (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 Calanz in their young free-swimming stage, 
in the Firth of Forth, occurred during April and May, and 
