CH. VIl] THE DISTRIBUTION OF THE PLANKTON 145 



area, say twenty miles apart, unless perhaps in the open ocean. 

 Then the plankton is hardly ever the same at all levels of the sea 

 and Ave almost invariably find that the catches obtained from 

 different depths vary considerably. So familiar a fact is this that 

 hosts of nets have been devised to fish only at definite depths of 

 the sea, and any one who has used these nets will know how 

 different the catches made at different strata may be. 



Stratification of the plankton. The most careful and 

 rigorous experiments of this kind with which I am acquainted are 

 those which were made by Lohmann^ in the open Mediterranean 

 off Syracuse. Lohmann made nine catches during the period 

 May 1 — 11, 1901. Each catch was made at the same time — between 

 7 and 8 in the morning. All the catches were made at the same 

 place, and they were made by sinking a closing water-bottle down 

 to a different depth on each occasion and so obtaining a water 

 sample from the stratum to be studied. One quarter-litre of this 

 water was filtered through thick taffeta silk, and the catch so 

 obtained was examined alive so as to avoid the loss of organisms 

 due to the destructive action of the preservative which would 

 otherwise have been used. The separate individuals of the micro- 

 plankton (protozoa and protophyta) in the catches were all counted. 

 The results shewed that most of the species obtained were more 

 abundant at a depth of about 40 metres from the surface, and that 

 the abundance of the plankton decreased slowly from that water 

 level up to the surface, and rapidly down to the bottom. 



Heliotropic migrations. There is thus a "stratification" of 

 the plankton. It usually varies both in nature and abundance 

 with the stratum of sea investigated ; and this vertical irregularity 

 changes rapidly from time to time. Now in accounting for the 

 causes of this vertical distribution we may with some probability 

 assign two factors : (1) the variations in the amount of light falling 

 on the surface of the sea and penetrating down to the deeper 

 layers ; and (2) variations in the temperature of the surface 

 water. There is little doubt that the variation of the light is a 



^ '^Neue Untersuchungen.'" Table I. gives the results of these trials. 

 J. F. 10 



