266 



THE SEAS 



to the surface (Fig. 55). In this way can be found the 

 actual depth at which the different kinds of organisms live. 

 But besides the plankton organisms caught by the tow 

 net, there are many forms so minute that they pass through 

 the meshes of the finest silk obtainable. Their presence 

 in the sea was first shown by an examination of the stomach 

 contents of those small planktonic tunicates, the Oikopleura, 



WEIGHT. 



RELEAilNq APPARATUS. 



. WEJQHT. 



Fig. 55, — A closing tow net ; A, fishing ; B, closed. 



whose houses and fishing apparatus were described on page 

 199. Many forms were discovered there which had never 

 been seen before in any of the tow net catches, so that a 

 new method had to be devised to obtain them in sufficient 

 quantity to throw light on their distribution and importance. 

 It was found that this could be done if a sample of water 

 was centrifuged in a small pointed tube. Samples of water 



