140 QUANTITATIVE PLANKTON INVESTIGATIONS [PART II 



long and powerful tail of the Oikopleura. The animal hangs on to 

 the internal filtering apparatus by its mouth and from time to 

 time it sucks out the organisms which have been retained by the 

 filter and eats them. Soon the meshes of this filter become choked 

 up, and when this happens the house is thrown off and in a short 

 time a new one is secreted. No one house is used for longer than 

 about six hours. 



Since the meshes of the second, internal filter are much finer 

 than those of the outside grating it happens that organisms which 

 pass through the grating are retained by the internal filter. 

 Generally only such organisms as are less than SO/jl in diameter 

 pass the grating. Thus Ceratia and most radiolaria, and the larger 

 diatoms such as CJiaetoceros, Rhizosolenia and Bacteriastrum do 

 not enter the house, and the only organisms that do so are the 

 smaller diatoms, the smaller protozoa and many of the smaller 

 protophyta, and it is only these that are to be found inside the 

 house. 



Now the area of the excurrent spout can be measured and the 

 flow of water through it can easily be estimated. Lohmann thus 

 found that about 162 c.c. of water passed through the house of a 

 large Oikopleura in about six hours. But all the micro-plankton 

 which was contained in this volume of water was not to be found 

 in the internal filter at the end of this period, for obviously some 

 of it had been eaten by the animal. Probably the catch contained 

 in the house could be taken as that originally contained in about 

 100 c.c. of water. If then it was possible to kill the animal after 

 the filtration had been carried on for some time it would be 

 possible to estimate the quantity of micro-plankton which had been 

 filtered from an approximately known volume of water. This was 

 easily done, for the internal filter of the Oikopleura house could 

 be dissected out and its contents examined and counted. When 

 this was done the results were very surprising. All the organisms 

 caught by the Oikopleura were such as would easily have passed 

 through the meshes of the silk used in the Hensen nets, and 

 many would even have passed through the much closer meshes 

 of the taffeta silk ; yet even in a volume of less than 100 c.c. of sea 

 water there are obviously a surprising variety and abundance of 

 planktonic life. 



