30 



moCEKDIXGS OF THE 



were abuadant, and nearly all the gatherings bad a greenish tinge. 

 Durino' that period the plants were more abundant in the bottom 

 waters, and the animals at the surface. 



Day after day we found that the two closing vertical nets hauled 

 up from 20 to 10 fathoms were of a brownish-green colour and 

 contained (especially the jSTanseu) an abundant gathering of Diatoms. 

 The surface nets during this time contained more Copepoda. On 

 April 15th and 19th, however, when the change in plankton was 

 taking place, the Diatoms were found to be mainly on the surface 

 and the Copepoda below. As an example of wide distribution I 

 may cite April 10th, when the nets gave consistent results all the 

 afternoon at three localities north of Port Erin, the Diatoms being 

 in all cases more abundant at the bottom and the Copepoda on 

 the surface. 



AVe were fortunate enough on one occasion to obtain incontro- 

 vertible evidence of the sharply defined nature of a shoal of 

 organisms, forming an instructive example of how nets hauled 

 under similar circumstances a short distance apart may give very 

 different results. On the evening of April 1st, at the " alongshore" 

 station III., north of Port Erin, off the " Cronk " one mile out, I 

 took 6 simultaneous gatherings in both surface and deeper waters. 

 Two of the nets were the exactly similar surface townets which I 

 have called B and C. At half-time, as the result of a sudden 

 thought I hauled in B, emptied the contents into a jar, and 

 promptly put the net out again. This half gathering was of very 

 ordinary character, containing a few Copepoda, some Diatoms and 

 some larvae, but no Crab Zoeas. At the end of the 15 minutes, 

 when aU the nets were hauled on board, aU the gatherings, in- 

 cluding B, showed an extraordinary number of Crab Zoeas render- 

 ing the ends of the nets quite dark in colour. B was practically 

 the same as C, although B had only been fishing for 7 minutes. It 

 Avas evident that at about half-time the nets had encountered a 

 remarkable swarm of organisms which had multiplied several times 

 the bulk of the catch and had introduced a new animal in enormous 

 numbers. Had it not been for the chance observation of the 

 contents of B at half-time, it would naturally have been supposed 

 that, as all the nets agreed in their evidence, the catches were fair 

 samples of w^hat the water contained over at least the area traversed 

 — whereas we now know that the Zoeas were confined to, at most, 

 the latter half of the traverse and may have been even more 

 restricted. Under these circumstances, an observation made solely 

 in the water traversed during the first 7 minutes would have given 

 a very different result from that actually obtained ; or, to put it 

 another way, had two expeditions taken samples that evening at 

 what might well be considered as the same station, but a few 

 hundred yards apart, they might have arrived at very different 

 conclusions as to the constitution of the plankton in that part of 

 the ocean. 



The bearing of such observations as these upon some recent 



