Rees (1949) has established significant biological differences between the two forms of the copepod Calanus finmarchicus, namely finmarchicus and helgolandicus. It is therefore necessary that in future research on the behaviour of Calanus the two forms should be distinguished.

I have myself published observations on the vertical distribution of Calanus and on the seasonal changes in size of individuals of the populations of this copepod off Plymouth. In order to know whether my results have been vitiated by the presence of the two forms in the collections, I have re-examined representative samples of adult females, using the fifth leg as the distinguishing character. The collections in question were those made in 1926 which formed the subject of a paper on seasonal changes in vertical distribution and in the sizes of individuals of different generations (Russell, 1928); and those made in 1931 and recorded in my paper on the vertical distribution of Calanus in relation to light intensity (Russell, 1934).