164 ANNALS OF SCOTTISH NATURAL HISTORY 



history, and gave them a detailed account of the observations 

 that had been made. I 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 en 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 larvse of insects " freshwater shrimps " (Gammarus), 



