164 ANNALS OF SCOTTISH NATURAL HISTORY 
history, and gave them a detailed account of the observations 
that had been made. TI had the privilege of reading Mr. 
Tullis’s narrative, and later on he himself favoured me with 
- some additional notes, and from these I obtained the informa- 
tion embodied in the preceding remarks. 
The phenomenon observed by Mr. Tullis and described 
in the communication to his friends suggests one or two 
questions relating to the micro-crustacea, that, for several 
reasons, appear to be of some importance, and which I 
propose to notice in the remarks which follow. 
One of these questions, and perhaps the foremost in 
importance, is the place that the micro-crustacea—or entomo- 
straca, as they are usually called—hold in the fishes’ “ bill of 
fare”; or, in other words, their value as fish-food. That the 
entomostraca hold an important place in this respect seems to 
be undoubted ; for not only do they occur in vast numbers, 
but when healthy and vigorous they are rich in a kind of oily 
matter that is more or less diffused through their tissues, and — 
it is owing to the presence of this oily matter in their bodies, 
as much as to their numbers, that, as fish-food, they are so 
nutritious and valuable. | 
The entomostraca when collected together ex masse 
soon die, and the oily matter in their bodies is set free and 
collects in the form of globules within the carapace, or it 
escapes and rises to the surface of the water and forms a 
perceptible scum. When strong and vigorous, this oily 
matter exudes from their bodies even during life, and hence 
it is—as sailors know well enough—that during summer, 
when the sea is teeming with small crustacea and other 
minute animals, a film of oil may frequently be seen spread 
over considerable areas of the surface of the water, so that 
during stormy weather the waves do not rise so high nor 
break so readily as during the winter months, when minute 
crustacea and other organisms are usually less abundant and 
vigorous. 
But though the entomostraca are unquestionably of great 
importance as fish-food, and specimens of various kinds are 
frequently obtained in the stomachs of fishes, it cannot be 
gainsaid that, as regards trout and other freshwater species, 
the larve of insects—* freshwater shrimps” (Gammarus), 
