MARINE BIOLOGICAL STATION AT TORT ERIN. 45 



These are all obvious factors in the problem, and the 

 constitution of the Plankton from time to time throughout 

 the year must be due to their interaction. The difficulty 

 is to disengage them from one another so as to determine 

 the action of each separately (See fig. 17 which illustrates 

 the effect of the above factors). 



Figure 18, on Plate IV, shows the Nauplius stage of 

 the common rock barnacle (Balanus) . The adult is present in 

 abundance on the rocks at Port Erin, and the Nauplii are 

 sometimes found in enormous swarms in spring. Figure 

 19 shows the succeeding or " Cypris " stage, found in our 

 tow-nets a little later and in much smaller quantities than 

 the Nauplius — the reduced numbers in the later stage of 

 the life-history being evidence of the action of enemies 

 and other factors of the environment. 



Amongst the physical conditions coming under the 

 third heading, the temperature of the sea is usually given 

 a very prominent place. I shall allude here to one aspect 

 only of this matter. 



It is often said that tropical and sub-tropical seas are 

 relatively poor in Plankton, while the colder Polar regions 

 are rich. In fishing Plankton continuously across the 

 Atlantic it is easy from the collections alone to tell when 

 the ship passes from the warmer Grulf Stream area into 

 the colder Labrador current. This is the reverse of what 

 we find on land, where luxuriant vegetation and abundance 

 of animal life are characteristic of the tropics in contrast 

 to the bare and comparatively lifeless condition of the 

 Arctic regions, Brandt has made the ingenious suggestion 

 that the explanation of this phenomenon is that the higher 

 temperature in tropical seas favours the action of 

 denitrifying bacteria, which therefore nourish to such an 

 extent in tropical waters as seriously to diminish the 



