CH. Vl] QUANTITATIVE PLANKTOX INVESTIGATIONS 139 



one square metre in cross section then the result of the pump 

 method can easily be converted in that of the column of unit 

 dimensions. 



It is only with respect to the smaller organisms in the water 

 that the ordinary nets fail, and Lohmann investigated these by 

 means of filters made either of hardened paper or of the material 

 known as taffeta silk. This is a fabric of thick and close strands 

 which leave very fine interspaces between them. It was made 

 into filters of the ordinary conical shape, or it was tightly tied on 

 to the ends of glass tubes, and the catch was introduced into the 

 latter and was made to pass through the cloth by gentle pressure 

 with a little indiarubber bulb provided with an air valve, by means 

 of which air could be forced into the tube above the level of the 

 water to be filtered. Obviously much smaller samples are required 

 when the smaller organisms are to be collected, and Lohmann 

 found that even ^ litre would yield a sufficient quantity of water 

 to enable an estimate of the abundance of these things in the sea 

 to be made. 



Capture of plankton by appendicularians. Finally Lohmann 

 had the happy thought of employing the houses {Hduser) of 

 appendicularians to determine the contents of the sea so far as the 

 smaller protozoa and protophyta were concerned. Oikopleara 

 albicans is an appendicularian which forms a gelatinous house or 

 case of some dimensions. This structure serves as a means of 

 protection for the animal which lives inside it and also acts as a 

 filtering apparatus by which the ascidian obtains its food from 

 the plankton of the sea, and compared with it all our plankton 

 filters and nets are very crude contrivances. The house is 

 furnished with two openings. One is a kind of grating, the meshes 

 of which vary in size with the size of the house. In a house 

 which is 17 mm. long the meshes of the grating are about 

 127^6 X 34-5/Lt. 



This grating is the opening through which water enters into 

 the house of the Oikopleura. The latter is so constructed that 

 after entering it the water has to pass through a second, and finer 

 filter before it flows out to the sea again through a spout. The 

 cause of the current of water is the continual movement of the 



