SURFACE-SWIMMING FAUNA (INVERTEBRATES). 85 



moments the water that had been alive with 

 these animals seemed to contain not one. An- 

 other time, after dredging nearly all the after- 

 noon at Lulworth for Bormiphora, with the very 

 poor success of a half dozen specimens, the net 

 came up simply choked full of these little round 

 jelly-like Ctenophores, and for the remaining 

 hours of daylight there appeared to be an 

 abundance of them all along the coast. One 

 morning in the Tropics, at about an hour after 

 sunrise, I was looking over the side of a steam- 

 boat, and saw that the surface waters were 

 full of beautiful and rare species of floating 

 animals. In less than half an hour afterwards, 

 when a boat was put off, scarcely one of them 

 could be found. Anybody who is accustomed 

 to working with a tow-net can give similar 

 experiences. 



In each of these cases a simple explanation 

 might be suggested. In the first case it might 

 have been the change in the tide which effected 

 the disappearance of the Jelly-fish ; in the second 

 it might have been the approach of nightfall 

 that caused the Hormiphoras to rise; and in 

 the last case it might have been the approach of 

 the heat of day ; but when carefully considered 

 such explanations are not sufficient, in that they 

 do not account for the suddenness of the change. 



The fact is that the conditions of life in the 

 surface waters are so complicated that it is ex- 

 tremely difficult for us to accurately estimate 

 the balance of the forces which act upon these 

 organisms. The direct heat of the sun, the light 

 of both the sun and the moon, the tranquillity 



