1898.] PLANKTON or THE FAEEOE 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 he southern (warm-water) forms, driven by the 

 North Atlantic Drift into bigher 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 aud one 

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



(3) A diiferent explanation seems reasonable in the ease 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 that the destruction 

 of such a small surface population should produce dead spe- 

 cimens in such abundance and with such regularity in the deeper 

 strata. Euclueta norvegica, Metridia Tonga, and Pleuromma ahdo- 

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

 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 recordetl 

 in surface collections. 



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

 regularity both in Mesoplankton and Epiplanktou, it seems fail- 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, ii'or example, the list of the captures of 

 Calanus finmarchicus 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 obserA^ations are numerous 'enough and sufficiently 

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

 at all being drawn. Most mesoplauktonic specimens are dead when 

 they arrive inboard ; the sudden alterations of press'ure 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 Epiplanktou. 



a few. 

 Numerous, constant. Numerous, constant. Epiplankton and 



Mesoplankton. 

 None, or occasion- Numerous, constant. Mesoplankton. 

 ally a few. 



