1898.] PLANKTON OF THE FAEBOE CHANNEL. 579 



Channel, where hotter and colder surface currents are constantly 

 at war. 



(2) This explanation may probably be further extended to cases 

 such as those of the six Copepoda already mentioned (pp. 548-9) ; 

 they appear to be southern (warm-water) forms, driven by the 

 North Atlantic Drift into higher latitudes (colder temperatures) 

 than they can bear. Although southern forms, none of them were 

 taken at the surface in 17 hauls, five were captured once and one 

 twice in 13 Mesoplankton hauls ; all six were few in numbers. 



(3) A different explanation seems reasonable in the case of species 

 which are taken in numbers and with regularity at considerable 

 depths, but appear rarely or never at the surface (if at all, then 

 generally at night). It is to me inconceivable thai; the destruction 

 of such a small surface population should produce dead spe- 

 cimens in such abundance and with such regularity in the deeper 

 strata. Euchwta norvegica, Metridia longa, and Pleuromma abdo- 

 minale (pp. 543 and 547) are examples of this distribution ; they 

 seem to be forms which, at any rate in these latitudes, exhibit a 

 preference for a mesoplauktonic existence, but which can and do 

 exist at the surface also under certain circumstances. Two of the 

 species are Arctic type-forms, which in these latitudes seek deeper 

 (colder) water, and may perhaps eventually be taken very much 

 further south as Mesoplankton than they have as yet been recorded 

 in surface collections. 



(4) When a species is taken in equal abundance and with equal 

 regularity both in Mesoplankton and Epiplankton, it seems fair to 

 infer that it is eurythermal and eurybathic ; it does not seem 

 possible that all the deeper specimens are deep merely because they 

 are dead and sinking. .For example, the list of the captures of 

 Galanus Jinmarchicus on the * Eesearch ' (p. 542) seems to exclude 

 such a possibility. 



It seemed worth while to cite these instances of criteria, which 

 may be applied in dealing with collections of Plankton from various 

 zones, if the observations are numerous enough and sufficiently 

 near together in time and place to permit of any general conclusions 

 at all being drawn. Most mcsoplanktonic specimens are dead when 

 they arrive inboard ; the sudden alterations of pressure and tempe- 

 rature, and the damage by the net itself, are most fatal ; further, 

 decay is so retarded at low temperatures in sea-water, that not 

 even microscopical examination can be relied on as evidence of 

 the life or death of the organism at the moment of capture. The 

 criteria applied above may be expressed thus :- 



Specimens at surface Specimens below Species belongs to 



Numerous, constant. None, or occasionally Epiplankton. 



a few. 

 Numerous, constant. Numerous, constant. Epiplankton and 



Mesoplankton. 



None, or occasion- Numerous, constant. Mesoplankton. 

 ally a few. 



