MARINE BIOLOGICAL STATION AT PORT ERIN. 67 



bution I may cite April 10th, when the nets gave con- 

 sistent 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. 



We were fortunate enough on one occasion to obtain 

 incontrovertible 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, oif the kk Cronk " one mile out, I took 

 six simultaneous gatherings in both surface and deeper 

 waters. Two of the nets were the exactly similar surface 

 tow-nets 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 all the nets were hauled on board, all the 

 gatherings, including B, showed an extraordinary number 

 of Crab Zoeas rendering the ends of the nets quite dark in 

 colour. B was practically the same as C although B 

 had only been fishing for seven minutes. It was 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 what 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 



