1go1-1902.| The Entomostraca of Mid-Lothian. 31 
castle-on-Tyne,’ vol. xiii, Part 2., which is the form we 
ourselves have found. JD. galeata, Sars, as described by 
Lilljeborg in his ‘Cladocera Suecie,’ seems to differ consider- 
ably from the British form, inasmuch as the adult (summer 
form) is helmeted.) 
The variation that occurs with regard to the eye and eyespot 
is also suggestive. The presence or absence of one or the 
other, and their size and position both relative and actual, are 
taken as constituting generic characters. With regard to the 
three families, Bosminide, Macrothricide, and Lynceide, as 
described in Norman and Brady’s monograph, we find that in 
Losmina there is a large eye, but no eyespot; while in 
Monospilus the eyespot is present, but there is no compound 
eye. In other genera both eye and eyespot vary much in 
size and position; in Drepanothrix the eyespot is large and 
quadrangular; in Lathonwra it is a mere speck; in some of 
the Lynceide it is larger than the compound eye. Coming to 
the family Daphnide we find that the eyespot is present in 
Ceriodaphnia, Scapholeberis, and Simocephalus, and absent in 
Moina ; and that in Daphnia it is generally, but not invariably, 
present. How is this variation to be explained? It seems 
probable that a now functionless eyespot may be the degener- 
ate representative of a former simple eye, which at some stage 
in the creature’s history became superfluous and is now 
disappearing. This would account for its complete disappear- 
ance in some cases, and its insignificance in others ; and if it 
is no longer useful to the creature, and therefore no longer 
_ subject to the action of natural selection, we can understand 
} that it might in the process of degeneration assume the variety 
of forms now existing. 
, Collecting. 
‘ 
i Our method of collecting entomostraca is as follows: we 
_ have a stick about eight feet long, which can be unscrewed 
into two parts for convenience in carrying. To one end is 
_ fastened a wire hoop, which may be from six to twelve inches 
across, and on to which is sewn a fine muslin net. This 
tapers gradually towards the lower end, which, instead of 
7 being sewn up, is fastened round the mouth of a small wide- 
I necked bottle. Many collectors use the muslin net by itself, 
VOL, IV. Z 
